Also very useful information. Thanks.
I may run an a5 in this, I have one in the parts bin.
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As a training analog, not for ballistic prowess. A 10.5” or 11” would be perfectly fine, ballistically (as much so as any other pistol cartridge gun), and won’t rattle your fillings like a 5.56 version.
The 1/2” longer bit is because blowback barrels don’t need a barrel extension to house the bolt lugs, so a 5.56 barrel is externally a half inch longer than the same internal (ATF) length blowback 9 barrel. Sometimes manufacturers (like New Frontier) just cut the barrel longer than advertised to account for that. Most do not.
If you’re the type that likes a handguard to go all the way to the muzzle, that 1/2 inch will matter when you do the math. If not, it won’t.
I read that a good compromise of shortest barrel length and performance is 7.7". Part of the reason that I recently got a Scorpion 3 + with a 7.8" barrel. I have not shot it yet though.
Just ordered a BKing firearms BCG, weighing in at 14.6 oz, and a 7.5 oz KVP 9mm buffer. That brings the reciprocating weight to right around 22oz, which, according to Blowback9, is the desired weight for a 9mm rig….
Upper is arriving tomorrow. I’ll have to go start my paperwork on a new pistol lower, and I suppose A5 system is out of the question now. Will have to research what buffer spring I will run.
This leaves me with a partially non dedicated lower, as I can always bail on the 9mm and swap out the guts to run 5.56.
Endo mag guts will be next. Another reversible component. Thanks to all who’ve contributed.
I just did this. Put together a 9mm upper to swap out with a 5.56 upper with minimal fuss.
For the barrel and BCG, I went with a CMMG radial delayed barrel and bolt kit. It drops right into a standard upper. It was a little pricey at $350, but I also didn't need to buy any special buffers or springs either, and those add up. I'm running an H buffer and standard spring in my 5.56 rig and the 9mm runs perfectly as well.
For mags, I tried both glock and colt pattern adapters and they both worked well with ball ammo. I settled on ejectorless endomag inserts. They were the most reliable over a broader range of ammo types.
When I switch from 5.56 to 9mm it is literally a matter of swapping uppers. No mag adapters, no changing buffers or springs. One to the other in less than a minute.
If you go blowback, I ran a Vltor A5SH 3 or 4 buffer with a sprinco blue spring. Although the recoil was harsh, it ran very well. I also used the endomags with ejectors, but they are kind of delicate. They will quit working at some point.