Second look at blowback pistol cartridge carbine

[stagpanther]

I built a 10mm AR a couple of years ago--and after a few "teething issues" got in t running fine. One of the few advantages of the blowback set-up IMO is that I can swap out the barrel in about 60 seconds by simply spinning the hand guard off, swapping barrels, and spinning the hand guard back on. Recently, I bought a 9mm barrel and got to thinking--I wonder if this would run on the same set-up with nothing more than a bolt swap-out?

The answer is yes. There remains one issue, the Spartan lower requires Glock large frame magazines--and the 9mm isn't that. So that's my only technical roadblock at this point for making this a very useful quick-change set-up.

I know I've knocked the 9mm blowback AR before--but my jaw dropped today after I shot 2 groups of the cheapest Winchester roundnose FMJ I could find --and at 100 yds using only an unmagnified 4 MOA reddot I was amazed at the tightness of the groups. Rated by Winchester at 1190 fps--I was getting 1450 out of this. I think I'm going to have to do a bit more exploring with this.

[stagpanther]

Well, I just did some work on a large frame 10mm magazine feed lips and got it to work--just got back from firing off two magazines --including one rapid-fire--and no hangups or failures. I'm a happy camper--this should open up a lot of options for quick caliber change-outs.

[TruthTellers]

Could you describe the "teething issues" you had with the 10mm build? I'm down for a 10mm carbine that uses Glock mags, but nobody is making one atm and after building a 9mm AR, I do think the AR makes for a great PCC platform. So good that unless it folds in half like the Kel Tec does, I'm never buying a PCC other than an AR again.

I used the Stern Defense magazine adapter. So, yeah, in total the Anderson lower and adapter ended up costing me an extra $40-50 for the lower, but when all the stripped 9mm lowers are the same price as the Stern adapter I'd rather have a standard lower I can use if I want to build a .223 or .300 BLK in the future.

[stagpanther]

Could you describe the "teething issues" you had with the 10mm build? I'm down for a 10mm carbine that uses Glock mags, but nobody is making one atm and after building a 9mm AR, I do think the AR makes for a great PCC platform. So good that unless it folds in half like the Kel Tec does, I'm never buying a PCC other than an AR again.

I used the Stern Defense magazine adapter. So, yeah, in total the Anderson lower and adapter ended up costing me an extra $40-50 for the lower, but when all the stripped 9mm lowers are the same price as the Stern adapter I'd rather have a standard lower I can use if I want to build a .223 or .300 BLK in the future.

I bought my parts from Joebob outfitters--so I ended up using a Spartan lower which has the magwell configured for either large frame or small frame Glock mags--which is why I frigged with the 10mm mag to get it it to handle the 9mm bullets. Your approach sounds better to have an adjustable mag well--at least if it's your plan to try multiple calibers.

The "teething issues" for me came down to adjusting the ejector/BC rail blade so that the cartridges fed reliably from the Glock mag. In short, the mag introduces the cartridges at such an angle that if things weren't just right the cartridge would jam at an angle in the chamber. I also bought the last shot bolt hold open configuration--but after destroying a couple of bolt catch levers, not to mention the additional timing issues it introduced to the magazine position, I decided it wasn't worth the hassle and tuned it so that I could manually lock the bolt back but not with the last shot. Shoots pretty much trouble free now--including difficult flatnose or bullets like the extreme penetrator. That's just my experience--I've read other people have had no issues like mine so maybe I just goofed the assembly.

The quick caliber change-out is quite the thing--I'm surprised nobody in the industry hasn't offered this in a practical low-cost package--seems like they are more fixated on forcing you to buy an entire new gun for each caliber.

[TruthTellers]

I didn't buy the mag adapter to run multiple calibers, I just didn't want to be stuck with a lower that cost $200 and only worked for small frame Glock mags. I'd rather spend that same amount on a mag adapter and another $50 on a standard lower that I can use for pretty much anything.

If the pistol only AR lowers were $100, I would have bought that instead, but they're all over $160.

[stagpanther]

didn't buy the mag adapter to run multiple calibers, I just didn't want to be stuck with a lower that cost $200 and only worked for small frame Glock mags. I'd rather spend that same amount on a mag adapter and another $50 on a standard lower that I can use for pretty much anything.

If the pistol only AR lowers were $100, I would have bought that instead, but they're all over $160.

Most of the pistol caliber industry seems to be settled on the dedicated Glock magazine scenario--in fact I think a high percentage of the BCG's are predicated on the "funky" ejector/rail blade arrangement--so I'm curious how a conventional lower would avoid not needing special (expensive) mods to accommodate for that.

[9x19]

Welcome aboard.

I recently got a mag well adapter from Orion Guns and it has worked well, I have dedicated lowers of course, but far more standard models, so this is a more of a luxury item for me.

https://www.orionguns.com/9mm-40-cal...dapter_c25.htm

[stagpanther]

Very cool 9 x 19--I see the blade thingy is integrated into the well adaptor--but do they also make that for large frame calibers (45 acp and 10mm)? I did some work on a large frame mag and it seems to work fine with the 9mm bullets--I assume it would work with 40 SW as well.

[9x19]

I haven't really looked... I shoot very little .45ACP, and if I were to build a 10mm AR, I'd likely go DI and lose the fixed ejector.

For now my .450 Bushmaster satisfies my magnum pistol caliber AR itch.

[stagpanther]

I haven't really looked... I shoot very little .45ACP, and if I were to build a 10mm AR, I'd likely go DI and lose the fixed ejector.

For now my .450 Bushmaster satisfies my magnum pistol caliber AR itch.

I haven't shot a bushie--but I did build a 458 socom--none of these pistol cartridge AR's--including the 10mm fired full-throttle--come close IMO to the felt recoil. I'm not especially fond of the fixed ejector thing either--but have learned to make the best of it--the quick changeover from 9mm to 10mm is pretty cool for any gun IMO. Not even sure I know of one that you can do that easily with that's on the market already.

[TruthTellers]

Originally Posted by stagpanther

Most of the pistol caliber industry seems to be settled on the dedicated Glock magazine scenario--in fact I think a high percentage of the BCG's are predicated on the "funky" ejector/rail blade arrangement--so I'm curious how a conventional lower would avoid not needing special (expensive) mods to accommodate for that.

Seems that way, but there's good reason for going with Glock mags. Generally nobody, even non-Glock people, has anything bad to say about Glock mags.

One other advantage to using a magwell adapter over a dedicated pistol AR lower is if you register it as an SBR, you can use a sub 16 inch barrel for pistol calibers. That may make the difference for some people and I'm sure there's a sizeable number of people who have SBR'd lowers.

[stagpanther]

One other advantage to using a magwell adapter over a dedicated pistol AR lower is if you register it as an SBR, you can use a sub 16 inch barrel for pistol calibers. That may make the difference for some people and I'm sure there's a sizeable number of people who have SBR'd lowers.

I think it's a great idea and probably would have tried it--may yet--if I had known about them. But since I've succeeded in getting both small frame and large frame pistol calibers to work in my set-up I don't really care much. I also have a wide selection of handguns, so my interest in making a short-barreled pistol AR in a pistol caliber is nil. Instead, I'm interested in seeing what that extra 250 to 300 fps gets me in a carbine length barrel. Turns out, a lot. Glock mags are great--except they were conceived for Glock pistols and every problem I've ever had with jams in a carbine is due to the cartridge angle at the feed lip and the ability of the cartridge to feed at an appropriate angle into the chamber. It's not hard to tune the problem out--but it's an issue to contend with--especially in a bullet design that may hang in the chamber more easily than others. 9mm seems to feed easier than the 10mm, haven't had any jams with it other than the first few attempts at getting a large frame mag to feed a small frame cartridge.

[Nanuk]

Last year I built an AR pistol in 40 S&W, why 40 you ask.... Logistics. I had some teething pains to make it reliable with all ammo. It fed FMJ and HST's great but did not like truncated cone HP's. I installed a feed ramp for Taccom3G and then built up the ramp with JB weld, basically making a ramped barrel. If I had a machine shop I would machine one from Stainless. It will now feed empty brass.

The 8.5" BBL gets the 155 grn HST up to 1360fps. From what I understand the 9mm is very similar, past 8.5" velocity gains are SLOW.

[stagpanther]

The 8.5" BBL gets the 155 grn HST up to 1360fps. From what I understand the 9mm is very similar, past 8.5" velocity gains are SLOW.

That's true--but another 100-150 fps for a pistol bullet is still significant--given their relative ballistic inefficiency and how fast they loose energy. I have a couple of pistol AR builds--very cool and fun and all that--but the attendant gnashing of teeth and pulling out of hair over the brace and transgender changing between pistol and rifle is something to consider. Since I've reached the vaunted state of old fart geezerdom--blowing my hand off by inadvertently reaching out a bit long is a consideration too for favoring a plain Jane 16" carbine.

[TruthTellers]

The 8.5" BBL gets the 155 grn HST up to 1360fps. From what I understand the 9mm is very similar, past 8.5" velocity gains are SLOW.

6 or 8 inches longer of barrel for 20 to 40 fps more is not worth the added length to the gun or decreased rigidity.

The only rimless pistol ammo that this doesn't apply to are .357 Sig and 10mm. Velocity gains from 8 to 10 inches up to 16 inches is a bit more, around 60 to 90 fps more.

[Nanuk]

That's true--but another 100-150 fps for a pistol bullet is still significant--given their relative ballistic inefficiency and how fast they loose energy.

You are only going to see a gain of about 50fps for the extra 8" of barrel.

6 or 8 inches longer of barrel for 20 to 40 fps more is not worth the added length to the gun or decreased rigidity.

Exactly.

[stagpanther]

Quote:
That's true--but another 100-150 fps for a pistol bullet is still significant--given their relative ballistic inefficiency and how fast they loose energy.
You are only going to see a gain of about 50fps for the extra 8" of barrel.

Quote:
6 or 8 inches longer of barrel for 20 to 40 fps more is not worth the added length to the gun or decreased rigidity.
Exactly.
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I don't have a 8.5" barrel to compare with--I was running the numbers through QL. I'd be very interested to compare results using a known factory ammo just out of curiosity.

[44 AMP]

6 or 8 inches longer of barrel for 20 to 40 fps more is not worth the added length to the gun or decreased rigidity.

Decreased rigidity? of the barrel??? I don't think that matters much in a 9mm or 10mm carbine.

and, 20-40fps gain from 6 or 8 inches more barrel is not a fixed number. Each gun is going to be a bit different about that, and the specific loads used will make the difference even more.

9mm is loaded to be optimized from a pistol (or SMG) length barrel, and does not gain as much velocity per inch from a longer carbine barrel, as some other cartridges do. But it does gain some. The OP got about 250fps increase from his carbine, over the listed velocity for the load. That's a bit more than 20-40fps...

The only rimless pistol ammo that this doesn't apply to are .357 Sig and 10mm. Velocity gains from 8 to 10 inches up to 16 inches is a bit more, around 60 to 90 fps more.

"The only common rimless pistol rounds this doesn't apply to are..."

I have 3 that aren't .357 Sig or 10mm that gain considerably from longer barrels, though they aren't "common". .357 AMP, .44 AMP and .45 Win Mag.

[stagpanther]

Quote:
6 or 8 inches longer of barrel for 20 to 40 fps more is not worth the added length to the gun or decreased rigidity.
Decreased rigidity? of the barrel??? I don't think that matters much in a 9mm or 10mm carbine.

and, 20-40fps gain from 6 or 8 inches more barrel is not a fixed number. Each gun is going to be a bit different about that, and the specific loads used will make the difference even more.

9mm is loaded to be optimized from a pistol (or SMG) length barrel, and does not gain as much velocity per inch from a longer carbine barrel, as some other cartridges do. But it does gain some. The OP got about 250fps increase from his carbine, over the listed velocity for the load. That's a bit more than 20-40fps...

I think what he meant was that the gain over the 8.5" barrel was only 20 to 40 fps.

So guess what? I'm going to take identical barrels with the same bore and chamber from the same manufacturer but at the different lengths and test the difference with the same ammo just out of curiosity and see for myself. Might take a few weeks--but I promise I'll post the results whatever they may be.

QL--which admittedly tends to be sketchy with pistol cartridges--tells me I should get an 100 fps or more.In a pistol bullet that is not an insignificant difference in the world of terminal effectiveness out to 100 yds.

[Nanuk]

The OP got about 250fps increase from his carbine, over the listed velocity for the load. That's a bit more than 20-40fps...

The vast majority of the velocity is gained in the first 8" of barrel. For anything past that you gaining very little.

[stagpanther]

FYI--a rough reference http://www.ballisticsbytheinch.com/9luger.html

A typical pistol cartridge AR barrel is likely going to be vastly stronger than any pistol barrel--as well as providing uniform near full case support. I didn't build mine with the intention of shooting low recoil cartridges at 25 yds.

[TX Nimrod]

The vast majority of the velocity is gained in the first 8" of barrel. For anything past that you gaining very little.

This is quite true....and very untrue. I first noticed this 20 years ago with my Marlin M45 and my 1911. With mild loads the velocity increase was small, but with some high performance +P loads the differences approached 200 fps.

[Nanuk]

This is quite true....and very untrue. I first noticed this 20 years ago with my Marlin M45 and my 1911. With mild loads the velocity increase was small, but with some high performance +P loads the differences approached 200 fps.

What is untrue? What you are saying is correct that a carbine with a pistol cartridge will pick up 200-250 fps over a pistol. What you may no understand is that the majority of that velocity gain occurs in the first 8"- 9" of barrel, 3" -4" more than your 1911 has.

There will always be outliers, that is the nature of the game.

[44 AMP]

a carbine with a pistol cartridge will pick up 200-250 fps over a pistol.

Again, I must point out that this gain is dependent on the cartridge and load. Yes, most of the velocity can happen in the first 10" or so, again, dependent on the specific load. Slower powders vs faster powder makes a difference, in how much the gain is, and how much per inch of length.

I've seen .357 give 1670fps from a 6" and with the same ammo 2200fps from a Marlin carbine. (And be too hot for some guns! )

10" seems to be about the shortest barrel length to use if you want to get the best from burning 15-20gr of powder. Sure, more powder can be burned in a 10 or less tube, but a much higher percentage of it is just wasted.

The much smaller powder volume of the 9mm case and the fact that it is loaded with faster powders, less suited to carbine barrels means it gets less of an increase than a magnum round would from the same long barrel. Doesn't mean no increase, or that the increase isn't useful.

[stagpanther]

I think this little discussion comes down to "20 to 40 fps gain of a 16" barrel vs a 8.5" ' vs more of a gain, which I believe in a typical higher powered load will be more of a gain. My gut feeling is that if you start out with a reasonably fast moving 9mm bullet, the gain in velocity will be significantly more than 20 to 40 fps.

If your point is that there is a "diminishing rate of return" after 8.5" vs the rate of velocity gain from 4" to 8.5 inches--I would say absolutely. Is it therefore irrelevant and "useless" to use a 16" barrel? I definitely disagree with that.

In the end--my point is not whether the over-all gain in velocity in going with a 16" barrel is so significant it has major advantages over a pistol length barrel. However, given the VERY tight case constraints of the 9mm Luger (BTW, what kind of powder are you using to get 15 to 20 grs into that case?) cartridge, its' relative inefficiency, and the fast degradation of its' terminal performance as the range increase and velocity decreases--even a 100 to 150 fps gain is significant in the context of increasing range effectiveness and terminal performance. Obviously, using a heavier slower moving bullet from the get-go the gain in the extra length barrel will be negligible. But using better bullets with hotter loads (why else would you want a carbine over a pistol anyway?) that extra length will make a difference IMO.

This might be one of those "tastes great...less filling" kind of arguments.