I just completed a 16 in build to use for duty,SWAT.
I’m already getting the itch to do a SBR build and was wondering what thoughts were out there in reference to going with a 7.5 for duty/SWAT. Also I’m a K9 officer.
Thanks for any help.
Im by no means an expert, but from what I have gathered the 7.5 just robs the cartridge of too much. I actually think a 9mm AR performs better but dont take my word for it. Someone else will weigh in.
https://www.m4carbine.net/showthread.php?t=30876 You can look here. Some people were talking about the 7.5
I have a 16" Middie that I use for general patrol dutys. I run a 10.5 or an 11.5 for SWAT. The 11.5 is growing on me more and more but 10.5 is the absolute shortest I would go for a social rifle due to the ballistics of the 5.56
While I really like the concept of a 7.5 for serious use, there have been many ballistics tests that show how just a few inches off the barrel really rob the bullet from performing to its optimal potential.
a friend of mine has a Olympic Stubby w/ 7.5 barrel…he had a lot of “pigtail” gas tube broken :rolleyes:
I’d stick with something reliable like 10.5/11.5 for duty.
3 or 4 inch of difference are not worth the risk IMO
Unless you’re building something to be used strictly as an entry gun (and even then, the point is highly contentious), 10" is really the generally-accepted minimum barrel length for a duty weapon, with most preferring 11.5".
As stated, what you might gain in convenience with the reduced form factor is offset by what you clearly lose in ballistic performance and range – which sort of negates the purpose of going to a long-arm to begin with. That is somewhat less of a factor when you’re close-in and full auto (as you might be on an executive protection/security detail when concealment is non-negotiable), but obviously that isn’t going to be the case in an SBR.
In my view, the primary utiility of the 7.5" lies with its entertainment value, which is admittedly considerable. Just don’t expect to be taken too seriously if you show up with one in your patrol bag. If you are unfortunate enough to ever end up in a carbine v. carbine engagement, it will place you at a decisive disadvantage.
AC
I’ve seen some numbers from Wes at MSTN regarding the 7.5" Noveske Diplomat. He says the following velocities are produced from the 7.5" barrel:
XM193: 2364 fps
M855: 2244 fps
MK262: 2053 fps
By my calculations for XM193 (using the fantastic program Ballistic on my iPhone) that means at:
25 yards: 2277 fps with 633 ft-lbs of energy
50 yards: 2192 fps with 586 ft-lbs of energy
This is roughly the equivalent of the XM193 being used in a full length barrel (20"+?) with targets at 250 yards. I am not suggesting XM193 is the appropriate load for your purposes, i’m just using it as an example.
I have shot mine at 100 yards with 77gr BH and it has very good accuracy. While the velocity and resulting energy might be woefully low at that distance, i would think that 25 yards and in would still be effective. That said, your leaning toward the 11.5" options seems a wise choice.
I bought mine for a very specific use, where the overall length of the system is key. And i love it!
Would 6.8SPC or 7.62x39 be better for that barrel length, and also be available in an AR platform?
Starting to see more utility in that KAC PDW.
6.8mm definitely does better in short barrels, but I still think 7.5" is on the short side if we’re talking about a duty weapon. Some accomodation is always possible, but as a general rule, rifle cartridges don’t fare especially well in pistol-length barrels – nor should we really be surprised by this.
AC
Most of the 7.5" rifles I’ve seen have the Noveske KX3 attached for reliability purposes. Since the KX3 adds about two and a half inches to the length of the barrel, I’d rather start with a 10.5". My LMT 10.5" is very reliable, provided I feed it good ammo.
10.5 is the limit for me.
I have a friend who has a 7in diablo. it heats up too much. and that’s in semi auto fire.
hot enough that the xtm panels were too hot to the touch as well as the pistol grip. just imagine in full auto fire.
That’s what you get when you combine a SBR with a cartridge designed for an SBR.
Energy is not the issue here, fragmentation threshold is. This is a function of velocity, which the 7.5" just kills. Even Mk. 18 type SBRs (with a 10.5" or 10.3" 1/7 barrel) have a problem with generating enough velocity to cause fragmentation at anything but extreme close range.
IMHO anything below 10.5" in 5.56 is a toy.
When I was a baby my hair was soft and curly…Not like REALLY CURlY like wavy. But now its like 4a super curly and soft, so i really need the lace wigs or lace front wig, possibly full lace wig: http://www.royalmewigs.com
I understand, and i agreed a 10.5" or longer barrel is what the OP should be looking at.
I don’t know what velocity is generally regarded as the fragmentation threshold, but let’s say it’s about 2700 fps. If that’s the case, XM193 might drop below that velocity somewhere between 125 yards and 150 yards. Clearly 5.56mm is expected to have an effective range greater than this distance. My point was that a shorter barrel effectively simulates shots at greater distances, with ever decreasing effectiveness as the range increases and/or the barrel becomes shorter.
I see what you’re saying. Sure velocity is velocity whether it is a projectile at 150 yards from a 20" M16 or 2 yards from a 7.5" Diplomat (exaggerated a bit).
You have to understand that M193 and M855 out of a 14.5" M4 has lousy “fragmentation range” to begin with. I don’t remember off the top of my head but I want to say it’s below 200 yards. Obviously something like 75gr TAP extends this. Obviously “fragmentation velocity” is specific to a given projectile, and the beauty of heavy OTM rounds is their ability to fragment at a way lower velocity. Still though, I believe that velocity is 2700+ fps.
Don’t confuse effective range with “fragmentation” range. Effective range is the average distance a competent shooter can engage and hit a target at. The Army says it’s about 500 meters for an M4. Be aware you are NOT going to see fragmentation at this range.
The bullet at this kind of range will punch in one side and out the other like a knitting needle. Can this be lethal? Damn skippy. Is it ideal? Not so much.
Fragmentation range is the range at which the projectile will reasonably fragment when hitting a human target when fired from a given barrel length. I will try to find numbers to back this up…
Thanks for the posts, it’s what I thought, I just got a pretty good deal on a Noveske N4 10.5 with 10’ DD lite, Noveske complete bcg. Original owner fired a mag for function test and put it up.
Now its mine, and time to start with ATF.