Shortest barrel allowed on AR pistol?

[vdubber]

This is one reason my FFL03 is in the works. Just sucks it doesn't apply to newer era NFA items

Sent from my SM-N950U using Tapatalk

[sdv]

Is there a law on how short of a barrel you can put on a AR15 / AR9 pistol in Ca?

Legal restriction: < 16 inch

Practical restriction > 9.5 inch (55 grain will drop below 2700fps at 30 yards. 2700 is lowest level for good terminal ballistics, 2500 is the lowest level for reliable fragmentation, 2200 is the lowest level for tilting and fragmentation.)

Physical restriction of our universe: >= 0 inch

[sdv]

2700fps required for fragmentation of M193.
A 5.5" barrel makes right around 2000fps with M193 and even less with heavier bullet options.
Without fragmentation, a 5.56 is about equally effective as a 22magnum.
Both will make a 22 caliber pinhole but the 22magnum would probably actually expand more because it's a soft bullet instead of an FMJ...

I believe from the reports:
- Good terminal ballistics > 2700
- Reliable fragmentation > 2500
- Tilting and some fragmentation > 2200

So I played with ballistic calculator and few tables and got this

Legend
- Barrel, inch
- 55gr muzzle speed, fps
- 55gr critical distance, y (speed drops to 2700fps)
- 55gr fragmentation distance, y (speed drops to 2500fps)
- 62gr muzzle speed, fps
- 62gr critical distance, y (speed drops to 2700fps)
- 62gr fragmentation distance, y (speed drops to 2500fps)

Data

len		m55	c55	f55	m62	c62	f62
inch		fps	yrd	yrd	fps	yrd	yrd

10		2767	42	170			
10.5		2800	62	190	2612	-	72
11		2834	83	211	2644	-	93
11.5		2876	108	237	2675	-	112
12		2919	134	263	2707	4	132
13		2974	168	296	2766	41	169
14		3039	206	334	2841	87	215
14.5		3076	228	356	2861	99	228
15.5		3113	248	378	2881	112	240
16		3150	271	399	2904	125	254
16.5		3187	292	421	2927	139	268