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Posted: 12/19/2005 1:03:47 PM EDT
Which ammo are folks finding to be most reliable fragmentation-wise from a 7.3 bbl with a 1:7 twist? I would assume the 55-gr bullets would be better, correct? Thanks.

flcracker
Link Posted: 12/19/2005 2:07:52 PM EDT

Originally Posted By flcracker:
Which ammo are folks finding to be most reliable fragmentation-wise from a 7.3 bbl with a 1:7 twist? I would assume the 55-gr bullets would be better, correct? Thanks.

flcracker



Nothing will fragment reliably in a 7.5", regardless of wieght and twist. Even 77gr ammunition in 1 1/7 twist 7.5" will be below the fragmentation threshold IIRC. That is not to say it won't fragement, just to say that you can't count on it to...

Link Posted: 12/19/2005 3:18:16 PM EDT
[Last Edit: 12/19/2005 3:20:28 PM EDT by flcracker]
I guess I figured that the lighter bullet would be more likely to frag than the heavier - I was comparing 55-gr and 62-gr mil-surp in my mind. Are you suggesting that 77-gr would have a faster velocity? Don't quite understand your reference to 77-gr.

So, if the pistol were to ever be used for a non-paper target, should I be looking at the lighter hollow points, polymer ballistic tips, or some sort of pre-fragmented ammo? Thanks for the reply.

flcracker
Link Posted: 12/19/2005 3:55:56 PM EDT
<Nothing will fragment reliably in a 7.5", regardless of wieght and twist. Even 77gr ammunition in 1 1/7 twist 7.5" will be below the fragmentation threshold IIRC. That is not to say it won't fragement, just to say that you can't count on it to...>

Bull shit. I have done scientific backyard ballistics testing (j/k) on them. If you have a business purpose for your AR pistol, you want 75-77 grain ammo in it.

www.anathema-incarnate.blogspot.com

Pics are there.
Link Posted: 12/19/2005 5:49:04 PM EDT
[Last Edit: 12/19/2005 5:54:03 PM EDT by flcracker]
I checked out your link - thanks for the post. But I still don't understand why a heavier bullet (75-77 gr) would fragment more readily in flesh than a lighter (55 or 62 grain) bullet would - not too sure that phone books are a reliable substitute for ballistic gelatin or flesh . Please help me get past my mental hurdle here. Thanks.

flcracker
Link Posted: 12/19/2005 5:57:32 PM EDT
I believe it has something to do with more wieght being the reason, something to the effect of the heavier the bullet, the lower the velocity it fragments at(out of a .223).

Personally, if I wanted to use my AR pistol for those special "business" occasions, I'd be getting a 10"+ barrel(which I am).
Link Posted: 12/19/2005 6:02:51 PM EDT

Originally Posted By flcracker:
I checked out your link - thanks for the post. But I still don't understand why a heavier bullet (75-77 gr) would fragment more readily in flesh than a lighter (55 or 62 grain) bullet would - not too sure that phone books are a reliable substitute for ballistic gelatin or flesh . Please help me get past my mental hurdle here. Thanks.

flcracker



It's not about the weight - its about velocity and Bullet construction.
Suffice to say you will NOT get fragmentation or expansion from a 7.3" barrel ,
when using military type ammo, and shooting fairly soft material.
( I have a 7.5 " AR pistol)

If you MUST have an antipersonnel capability - you will need to try rounds
with "varmint" bullets , either hollowpoints or ballistic tips.
Link Posted: 12/19/2005 6:40:55 PM EDT
[Last Edit: 12/19/2005 6:41:35 PM EDT by bigcraig79]
AK you should check out my site. I got plenty of fragmentation from mine.

The evidence indicates that as the bullets increase in weight, they fragmetn more reliably.

100 grain bullets are supposedly like ebola lethal.

At any rate, my theory is based on the basic fragmentation principal. As the bullet tumbles, the cannelure (the part where it is crimped) is put under too much lateral pressure, and the bullet breaks apart.

I think this pressure is made worse the heavier the bullet is. Just think about it. The heavier the bullet, the more mass pressing against this weak point. The more mass pressing, the more readily it breaks, thus more fragmentation.

This is my physics geared explanation...

Then again I failed physics the first go round, but who knows.

Heavier bullets DO fragment out of a 7.5 inch barrel.

As far as the medium shot, it seems to me the denser the material, the more resistant it would be to complete fragmentation. I think a softer, gel (human body) type material would aid fragmentation, more so than a denser medium (books, mud, clay)
Link Posted: 12/19/2005 6:42:32 PM EDT
Thank you - that is what I was thinking would be the solution. Although this pistol will hopefully only ever be used for plinking, any firearm I own that could be used for self-defense always has at least two mags of the best anti-goblin ammo ready for use. My plan is to find a good ballistic tip or hollowpoint round that will work reliably in it, then find which mil-surp ammo shoots nearest to the anti-personnel round's POI for practice use.

I would be interested in hearing arguments for and against the 7.3-in bbl vs. the 10-in bbl. How much velocity difference are we talking about between the two? The velocity charts I've seen all bottom out at 16-in bbls. Thanks again, Sirs.

flcracker
Link Posted: 12/19/2005 6:57:36 PM EDT
Big craig79 - I was referring to mil issue ammo - 55gr - 62gr , I understand about the
"heavy" rounds.

flcracker - I am only guessing , but you are probably looking at UNDER 2500 FPS with even light
bullets from a 7.5"

I built mine as a 7.5" because it is after all supposed to be a pistol , not a short rifle,
and for me , its really just a Fun Toy.
Link Posted: 12/19/2005 6:57:51 PM EDT
[Last Edit: 12/19/2005 7:04:53 PM EDT by bigcraig79]
I think the velocity out of a 7.5 inch barrel is roughly 2300-2500 FPS assuming 55 grain ammo.

Heavier ammo might be down to say 2100-2300 range

AK, you are correct. I didn't have anything close to fragmentation out of the 55 grain lake city.

62 grain MAY get some fragmentation, but I wouldn't chance it, personally.
Link Posted: 12/19/2005 9:05:13 PM EDT
My understanding is the reason the 75-gr and 77-gr round fragement at lower velocities is that they still carry enough energy to do so. Energy = Mass x Veolicty X Veolcity and all that jazz. I also believe that the specific construction of the particular 75-gr and 77-gr bullets that are readily available has a lot to do with their fragemntation capability being in the 2400+ fps range instead of the 2700+ fps range like most the lighter stuff.

BigCraig- I had not seen any ballistic testing on the heavier ammo in short barrel ARs and in fact had asked several times for any info in the ammunition section of the forums. I am going to your page right now to see what I can learn!
Link Posted: 12/19/2005 11:15:15 PM EDT
If you plan on taking your short AR to a fight, you really need 8+ inches of barrel.

I know it does not seem like much, but the end of the violent acceleration is just past 8 inches. Beyond that, the powder has all burned, and it becomes more linear acceleration based on hot gas expansion.

Here is a graph

I cut a barrel off in 1 inch increments years ago, and got similar results to what they show on that chart.


As for bullet fragmentation, a lot depends on the target. Many times in combat, a bullet strikes something else before penetrating the torso. The arm is a common one, causing the bullet to yaw.... or the enemy's weapon / gear / etc.

Shooting ball ammo gives predictable results. A thin jacketed expanding bullet gives good performance on varmints, and thugs in tee shirts. It may not do so well if it strikes a sling and LBE first...


Lem

.
Link Posted: 12/21/2005 3:22:21 PM EDT
If you have a AR pistol as a anit zombie gun just remember, 2 in the chest and 1 in the head. Frag or no frag it still creates a hole, through cranium or vital organs.
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