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Which caliber for new ar-15 upper

  • .223 rifle

    Votes: 4 44.4%
  • 6.5 grendel rifle

    Votes: 2 22.2%
  • 9mm pistol

    Votes: 0 0.0%
  • .300 blk pistol

    Votes: 3 33.3%
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Discussion Starter #1
I Kinda panicked when i noticed some wear on my old ar rifle upper, so i ordered a new stipped upper just to have as a replacement. how knows what will happen with ar-15 parts availability in the future. so anyways, I have a nice carbine lenght 7" handguard that i took of my rifle, so im like 50% of the way to a complete upper! not quite but almost. Im not quite sure what to do with it though.

right now im leaning towards a 9mm pistol upper to put on my existing pistol multi-cal lower.
ar-9m barrels are pretty cheap, $100 or so. thing is i dont see many 9mm bcg's in stock. cmmg has a slick 9mm conversion kit with a 8" barrel and bcg, for $399, plus .223>9mm magazine conversion kits for like $75 a piece. but the nice thing is you can straight up use a standard ar lower, no mag well block needed to take in or out to use it. I was kinda hoping i could do this piecemeal, you know barrel here bolt there...

Or

I could go the other route and just get another rifle barrel and use this upper on my existing ar rifle lower.

i really want to try 6.5 grendel. I believe all i would need is a barrel, bolt and maybe magazine?

OR... just get another 5.56 barrel/ bcg. this would be most economical, and would also give parts interchangability between all my ar uppers and lowers.

that last option is actually pretty enticing. the 9mm would be fun. the 6.5 grendel would allow me to hunt and extend my reach, but I dont really need to do either. if shtf ill hunt with my .223 and hollow points.

what would you do with a blank upper?

PXL_20201105_031648382.jpg
 

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lol....why not all of the above.

If I didn't HAVE a .300BLK upper, that's where I'd start (all of these choices are predicated on the fact that you can actually get or make ammo to FEED the upper; that's a factor in the decision as well). It's fairly powerful, fairly versatile, and gives you more thump. It also can use existing magazines and BCGs. The .300 differs somewhat from other calibers in that it has decent burn characteristics out of short barrels--especially the subs (which can be thought of as a form of 9mm/.40 round but with better downrange accuracy). 9mm is certainly cheaper than 300BLK, but if you reload the 300 and have a decent source of this subsonics in a 300 function in the role of pistol caliber carbine. So while I can have a 300 BLK upper (and get a pistol caliber carbine if I want to go with the subs) and get both, you can't get both with the 9mm in that you can shoot supersonics though the 300 with rifle performance.

I've a 9mm PDW so really have no need for a 9mm upper at this point. But having a 9mm in a PDW role is a fantastic idea; if it's as part of an AR platform then that's great.

In order to exploit the ballistics of the 6.5 stuff you're going to need a longer barrel. Shorter barrels certainly can be accurate but do stuffer from velocity loss. That's one of the reasons I went with the 300 BLK option which seems to be the best of the bunch out of shorter barrels.
 

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Too tough to choose. The 6.5G is a lot of fun to play with. Better range than the others if you like shooting paper you can't see with the naked eye.

I have a 9mm ar pistol, but wouldn't have it if it didn't take the 32 round Glock mags....just my $0.02.
 

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I have them in 5.56, 300 both carbine and pistol and a carbine in 7.62 x 39. My 7.62 upper is the Midway house brand, AR Stoner. With the 150-154 grain ammo the 7.62 x 39 is far superior to all the others. Now wish I had not built the 2 in 300 BLK. You need to shoot them side by side with the same 16 inch barrels to really see the difference. Kind of like the difference between a 30-06 and 300 Win Mag. Mine is 100% with the AR Stoner magazines, not one failure to feed or eject ever. That was a problem a decade ago, but not now. The 7.62 seems more accurate than the 300 as well even with the cheap Russian ammo.

I have had two 9mm carbines. Not in the same class at all. I have the Keltec Sub 2000 that I carry in a backpack with my Glock 19, they take the same magazines. Fun to shoot but really just a 50 yard gun. The 9mm only gives you about 20% more velocity, so not really much increase compared to the rifle rounds.

FWIW
 

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Discussion Starter #6
thanks for the input!

I agree 9mm is more a fun gun than anything else.

6.5, while neat, I really dont need and probably wouldnt shoot alot.

.300 seems neat because its all the same parts as 5.56 except the barrel, but it does have a bit more thump than the 5.56 and 9mm. i like that there is a wide range of loadings to choose from. and, maybe if i do a .300 blk it will help convince me that i need a suppressor.

5.56 would be nice from a absolute compatibility standpoint, but i really dont need another 5.56 gun.

for context, I already have a 5.56 rifle and pistol upper/lowers.

personally im leaning .300blk.

x39 has its merrits, but, idk, doesnt really do it for me for whatever reason. not very much compatibility there.

keep em coming!
 

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Mosin? You did leave out the 7.62x39! That is a goodin'

9mm is more fun and cheaper these days to shoot!?!
 

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thanks for the input!

I agree 9mm is more a fun gun than anything else.

6.5, while neat, I really dont need and probably wouldnt shoot alot.

.300 seems neat becuase its all the same at 5.56 except the barrel, but it does have a bit more thump than the 5.56 and 9mm. i like that there is a wide range of loadings to choose from. and, maybe i do a .300 blk it will help convince me that i need a suppressor.

5.56 would be nice from a absolute compatibility standpoint, but i really dont need another 5.56 gun.

for context, I already have a 5.56 rifle and pistol upper.

personally im leaning .300.

x39 has its merrits, but, idk, doesnt really do it for me for whatever reason.
I built the 300 pistol thinking it would be really cool and it is ok. But compare to the 7.62 is like going from a 22lr to a 22 short. Just depends on what you want. For me the 300 is a personal defense gun. The 7.62 is what I put in the RV if going to bear country, do not believe the ballistic tables, they use long barrels and do not apply to the carbines and pistols.
The 7.62 by the way has several sub sonic factory rounds available now. I just like the extra power. If it is just for personal defense or just for fun, then go with the 300 BLK. If you are going to use it to hunt, the 7.62 x 39 gives you an extra measure of thump. I have only shot one big deer with it, but got full penetration and a 2 inch exit whole. I shoot both of them suppressed as well as the 5.56 and the 9mm. All are hearing safe but still pretty loud. The 7.62 x 39 is also cheaper ammo. Ammo seek has 80 entries for 300 BLK ammo, the cheapest Russian stuff starts at $1 per round and goes to $3.25 PER ROUND. https://ammoseek.com/ammo/300aac-blackout. They have 71 entries for 7,62 x 39 ammo starting at 35 cents for the Russians stuff and going to $1.50 per round. https://ammoseek.com/ammo/7.62x39mm.

So, if going with the 300 probably need to plan on reloading as those prices are nuts. FWIW
 

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I have several in 5.56 and 223. Also in 7.62x39 and don't underestimate that round. I also like the 6.8 SPC I have and my 458 SOCOM,
 

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9mm AR15s are sort of silly.

They also recoil more than 5.56 and almost all other actual dedicated pistol caliber carbines. The DI system and 9mm just don't go well. That's why the original 9mm Colt SMG's (which were 9mm AR15s) never really gained any favor with the military and why the pistol caliber AR15s only came at the tail end of the recent PCC resurgence.

If you want a 9mm gun, there are PLENTY of dedicated PCCs out there running the gambit of prices depending on how much you want to spend. Some like the CZ Scorpion being very reasonably priced and highly customizable with aftermarket parts much like the AR15.

TXPlt went the CZ Scorpion route and has one if you have any questions and he loves his.

I went the Sig MPX route and the B&T (Brugger and Thomet) Route and likewise am very satisfied with mine.


If you want a general "Do all" multi purpose built get another .223/.556. Its not going anywhere and its still the easiest to get ammo and parts for despite parts shortages.

If you want a superior self defense round for 200 yards and in, consider the .300 blackout. Its far superior to the 5.56 with supersonic ammo (within 200 yards)... (and if its past 200 yards you probably arnt justified in shooting someone anyways).

Only problem is with the ammo shortage... good luck finding any ammo for it.
.300 blackout has become wildly popular much like the 5.56 but with only a fraction of the ammo production... which under normal times would be fine but currently... its one of the hardest calibers to find currently.

Longer range or hunting, obviously there are the other 6.x calibers or more niche calibers.

So just depends on what you want.
 

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Discussion Starter #11
I built the 300 pistol thinking it would be really cool and it is ok. But compare to the 7.62 is like going from a 22lr to a 22 short. Just depends on what you want. For me the 300 is a personal defense gun. The 7.62 is what I put in the RV if going to bear country, do not believe the ballistic tables, they use long barrels and do not apply to the carbines and pistols.
The 7.62 by the way has several sub sonic factory rounds available now. I just like the extra power. If it is just for personal defense or just for fun, then go with the 300 BLK. If you are going to use it to hunt, the 7.62 x 39 gives you an extra measure of thump. I have only shot one big deer with it, but got full penetration and a 2 inch exit whole. I shoot both of them suppressed as well as the 5.56 and the 9mm. All are hearing safe but still pretty loud. The 7.62 x 39 is also cheaper ammo. Ammo seek has 80 entries for 300 BLK ammo, the cheapest Russian stuff starts at $1 per round and goes to $3.25 PER ROUND. https://ammoseek.com/ammo/300aac-blackout. They have 71 entries for 7,62 x 39 ammo starting at 35 cents for the Russians stuff and going to $1.50 per round. https://ammoseek.com/ammo/7.62x39mm.

So, if going with the 300 probably need to plan on reloading as those prices are nuts. FWIW

you have some great points, cost and availability mainly.

from my research super sonic .300 blk from a rifle barrel is very close to x39 in terms of ballistics.

if x39 used the same bolt and mags, it would be more appealing.
 

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I forgot to mention that the 9mm is a short round, a short bullet so to speak. In a handgun it will start about the speed of sound, 1100-1150 or so fps. In a carbine, it will start at say 1300 fps or a little more. What happens with the carbine is that out to about 50 yards all is well. But as those bullets slow down to the speed of sound, being short or of low ballistic co-oefficient they will yaw as the hit the speed of sound at shorter ranges. One of ours would shoot 1 hole groups at 50 yards, at 75 they went goofy as the bullet slowed down and were about 8 inches at 75. In 9mm you have to really find the bullet that has the best speed for it to remain stable out to say 100 yards, at which distance it will be dropping about a foot anyway and not much value. Or you can just use the 147 or 158 grain bullets which are always below the speed of sound and avoid that issue.

In the rifle rounds not a big deal, because all your rounds are above 1,800 fps and well above the speed of sound out to maybe 250 yards. In other words, the 300 BLK will be supersonic out to 350 yards but will drop about one foot at 200 yards which would be your effective range. So, the yaw that happens with the short 9mm bullet is never is an issue. You are just shooting a short rifle. Lots of issues with the 9mm AR or carbine. They are fun, just a very short range tool. I do recommend the KelTec Sub 2000. It folds down to 16.25 inches and fits my backpack, with a 33 round mag and my Glock 19 on the hip, I am pretty well armed on a hike. We carry it in the RV. On the other hand, I built the 300 BLK pistol for the RV and it just works well. Problem is it does not fold down for easy carry like the Keltec so I do not like to leave it in the RV when we go on a hike or play tourist. Nobody wants to come back to camp and find some bad guy holding your cool, AR pistol. FWIW

Let us know what you decide then follow up and tell us how it worked out for you. Accuracy info is helpful also.
 
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you have some great points, cost and availability mainly.

from my research super sonic .300 blk from a rifle barrel is very close to x39 in terms of ballistics.

if x39 used the same bolt and mags, it would be more appealing.
http://www.ballistics101.com/7.62x39.php
http://www.ballistics101.com/300aac_blackout.php
The official published ballistics from Ballistics 101 says that the 300 BLK is exactly 90% of the power level of the 7.62 x 39 with the 154/155 grain bullets. Kind of like comparing a 308 to a 30-06, not a lot. With traditional 125 grain bullets the 7.62 gets 200 fps more. Again not a lot, but almost exactly 10% more. If your shoot subsonic, then they are all downloaded to the exact level so that does not matter.

Your point about the bolt carrier group is certainly an issue, because they do not interchange. I have a pile of AR mags, so I have to keep them separate from the 7.62. Midway usa sells the 10 round mags for $15 and the 30 round ones for $20, so not a big deal to me. I only need about 5 big one and 3 little ones for travel to communist states. If we have a shooting war then I go back to my 5.56 guns.

Love these discussions, I love carbines, have several and never get bored with them. I have several levers also, but that is another topic.
 
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Discussion Starter #14
Well, I did it!

I just ordered a 7.5" .300 blk barrel, 1/8 twist.

call me crazy! I just sounds from what i can imagine will be really cool!

ammo's is a bit pricey, but, my 10.5' 5.56 is for blasting at the range. this one will be better suited for home defense and defense from critters while out in the mountains. even with the lower velocity the .300 still thumps harder than the 5.56 out of a short barrel.

so ive got an a2 birdcage coming as well, 5/8x24.

I already have a Gas block.

just need the gas tube.

Ill pick up a new BCG and bolt alittle later. but with what i've got coming it'll be enough to put it all together!

Super excited!

I've always wished my 10.5' was alittle shorter, so this is about as short as it goes!

I'm planning to run irons on it as ill use it 100 yards an in mostly. small and light is the key here. and wayy better than a handgun for larger wildlife.

It's from bear creek, they are $100 free shipping right now.
 

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Well, I did it!

I just ordered a 7.5" .300 blk barrel, 1/8 twist.

call me crazy! I just sounds from what i can imagine will be really cool!

ammo's is a bit pricey, but, my 10.5' 5.56 is for blasting at the range. this one will be better suited for home defense and defense from critters while out in the mountains. even with the lower velocity the .300 still thumps harder than the 5.56 out of a short barrel.

so ive got an a2 birdcage coming as well, 5/8x24.

I already have a Gas block.

just need the gas tube.

Ill pick up a new BCG and bolt alittle later. but with what i've got coming it'll be enough to put it all together!

Super excited!

I've always wished my 10.5' was alittle shorter, so this is about as short as it goes!

I'm planning to run irons on it as ill use it 100 yards an in mostly. small and light is the key here. and wayy better than a handgun for larger wildlife.

It's from bear creek, they are $100 free shipping right now.
I do not recall if you reload, but if you do a suggestion. I bought a bunch of 147 grain military ball ammo from Midway on the cheap 23 cents per. With 110 or 296 powder, the 300 only needs about 17 grains of powder, about 8 cents. A cost of 23 cent per bullet 8 cents powder and 4 cents per primer you can load them for 35 cents each and since the 300 is a low pressure round my cases so far seem to last forever. That beats the $1- $1.50 per round for ball ammo by a bunch.

You will need some sort of sling to carry the AR pistol. I just let my hang across my chest but they are pretty quick to pull up into action. I mention that because I see you are in Colorado. I have walked through areas in the dark where I have seen bears earlier in the day with a flashlight in one hand and a handgun in the other. My little 300 is far better. I might suggest one of the cheap small lights to mount directly under the barrel. On mine I have the popup irons but I also have a scope mounted with the little quick release levers. I leave the scope on during the day and pop it off at night. If I want to lay it on a limb and take a long shot at a coyote, no big deal, I can do that with the scope on it. Or just pull it off in 2 seconds.

Mine is the 10.5 inch that I also suppress. You will find there is a lot of versatility with them.

Fun and 9 kinds of functional. Enjoy.
 
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I do not recall if you reload, but if you do a suggestion. I bought a bunch of 147 grain military ball ammo from Midway on the cheap 23 cents per. With 110 or 296 powder, the 300 only needs about 17 grains of powder, about 8 cents. A cost of 23 cent per bullet 8 cents powder and 4 cents per primer you can load them for 35 cents each and since the 300 is a low pressure round my cases so far seem to last forever. That beats the $1- $1.50 per round for ball ammo by a bunch.

You will need some sort of sling to carry the AR pistol. I just let my hang across my chest but they are pretty quick to pull up into action. I mention that because I see you are in Colorado. I have walked through areas in the dark where I have seen bears earlier in the day with a flashlight in one hand and a handgun in the other. My little 300 is far better. I might suggest one of the cheap small lights to mount directly under the barrel. On mine I have the popup irons but I also have a scope mounted with the little quick release levers. I leave the scope on during the day and pop it off at night. If I want to lay it on a limb and take a long shot at a coyote, no big deal, I can do that with the scope on it. Or just pull it off in 2 seconds.

Mine is the 10.5 inch that I also suppress. You will find there is a lot of versatility with them.

Fun and 9 kinds of functional. Enjoy.
thanks for the info!

I do not reload but maybe this is what finally gets me into it. I wouldn't need to load alot, so a simple press set up would probably suffice. this is the first really "odd ball" caliber i've ever owned so its never been hard to find cheap and plentiful ammo in the past, save for a few times like now.

I'll definitely pick up as much of the .300 brass as i can manage to find.

I do have a light to mount on it, and I could throw on a RDS or a 3x fixed prism sight but we'll see. a sling is no problem to set up and i do that for every ar.
 

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Out of the calibers listed I would stick with 223. The 6.5 sounds nice but unless the military were to adopt it I doubt I will ever go there. At this point In the game to be honest I wouldn't stray outside of anything common.

I'd also stay away from 9mm, just because (as has already been said) it's too short ranged and there are several purpose built PCC's that are better suited.

300 might be a good option, but I opted for x39 instead because ballistically it's similar, much cheaper to feed and easier to find ammo for. With the biggest drawback being needing different magazines which have been generally easier to find than they used to be (current situation not Included) and reliable. Enough so that my last build was another x39 with an 18" bbl. Instead of a 300.

All that being said maybe a 223 of a different flavor than what you already have? That's one of the nice things about AR's.. You can have a basic carbine, a close in pistol variant or a full rifle variant that is much more pleasant to shoot. Have to say that I was very pleasantly surprised with the difference going to the longer barrel and gas system.. It was like shooting an entirely different rifle.
 

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Out of the calibers listed I would stick with 223. The 6.5 sounds nice but unless the military were to adopt it I doubt I will ever go there. At this point In the game to be honest I wouldn't stray outside of anything common.

I'd also stay away from 9mm, just because (as has already been said) it's too short ranged and there are several purpose built PCC's that are better suited.

300 might be a good option, but I opted for x39 instead because ballistically it's similar, much cheaper to feed and easier to find ammo for. With the biggest drawback being needing different magazines which have been generally easier to find than they used to be (current situation not Included) and reliable. Enough so that my last build was another x39 with an 18" bbl. Instead of a 300.

All that being said maybe a 223 of a different flavor than what you already have? That's one of the nice things about AR's.. You can have a basic carbine, a close in pistol variant or a full rifle variant that is much more pleasant to shoot. Have to say that I was very pleasantly surprised with the difference going to the longer barrel and gas system.. It was like shooting an entirely different rifle.

great points!

I would have gone .223/5.56 if i didn't already have 2 of them (16" and 10.5" ).

plan is to make the .300blk my woods/ bear/ truck gun, the 10.5 my home defense gun, and the 16" my " long range set up" (relatively speaking).
 

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Discussion Starter #20
about to spend $125 for 100rnd 125 gr hpbt..ouch! but i figure whats a gun without ammo?!

plus this should last a while and provide brass for reloading. its PPU , which ive liked in the past in other calibers.
 
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