Reduced utility of .308 in shorter barrels?

[dorchester]

Shorten a .308 barrel enough and the velocity drops to a point where fragmentation/expansion is compromised and muzzle blast/flash can increase to quite high levels. Here's the question: at what point do you stop shortening a 7.62X51 and just go to an already short 7.62X39 rifle? I doubt (but do not know) if a 16 inch .308 will fragment an FMJ style round; perhaps at that point only expanding point rounds should be carried. So here's my REAL question: I see a lot of 16 inch .308 rifles on the market (one of them I own) and yes they are more handy than the full size guns, but... are we just kidding ourselves that we have a more powerful rifle for our pains? Assuming the lack of fragmentation of an FMJ, is the relatively small increase in penetration over 7.62X39 worth all the hassle? Would it be smart to limit 16 inch .308's to Noslers and other good expanding bullet designs only? Since I also have an 18 and 20 inch .308 to use this is more than just a theoretical question. Thanks for any insights folks. Signed, "Too many uppers dude"

[Gutshot John]

At shorter ranges it strikes me that that's not going to be an issue. Certainly you would lose fragmentation velocity sooner/closer but losing it altogether seems questionable.

At longer ranges it probably runs out of gas sooner but shot placement rules. There's a fellow here and on the FALfiles who can speak to it more intelligently than I can but based on his chrono measurements it averaged about 50fps per inch of barrel lost so from 21 to 16 inches you're looking at 250fps if memory serves.

Do you have good numbers on .308 fragmentation/velocity tables? I'd be interested in seeing what it has to say. You seem to be taking it as gospel that fragmentation drops off at a 16" barrel. Not that it doesn't, I just haven't seen the data for that.

All in all it depends on what your requirements are. If you dont' really intend on shooting beyond 100-200 yards the 7.62x39 is a fantastic cartridge. If you want 200-500 yards out of it the 7.62NATO has its virtues.

[SHIVAN]

Shooting 155gr, 168gr or 175gr .308 bullets with better BC's and better SD's will yield better results than a .311 bullet in most cases, even when velocities are similar.

I obviously defer to anything DocGKR might have as data, but when I reviewed this some time ago the 7.62NATO/.308WIN still possessed quite an advantage in energy, velocity, SD, and BD -- even down to 12.5" barrels.

EDIT: For a look at what a slow moving .308 will do, check out the performance of the 300 Whisper/Fireball. The 125gr 2100 fps load mimics a 7.62x39. Now add a lot of case capacity, and a longer and heavier bullet. Sounds like a winner over either the Whisper or 7.62x39.

[Burt Gummer]

With conventional .308 rounds, it is my understanding that it takes an 18" barrel to fully burn the propellant. Shorter than 18" and you increase muzzle flash. With the advent of the Mk 319 Mod 0, where the load is optimized in shorter 16" barrels, compact battle rifles will be much more practical.

I'm hoping this trend in caliber development (Mk 319 Mod 0) will spill over to the civilian/LE side with manufacturers. Maybe Hornady will produce a "short barrel" .308 round with 130 gr AMAX for their TAP line?

[Ga Shooter]

[QUOTE=SHIVAN;585264
I obviously defer to anything DocGKR might have as data, but when I reviewed this some time ago the 7.62NATO/.308WIN still possessed quite an advantage in energy, velocity, SD, and BD -- even down to 12.5" barrels.

.[/QUOTE]

I am currently torn between an precision bolt action and a Larue OBR. Along with my dilema is what barrel length. How much accuracy/power gets lost with shorter semi-autos as compared to longer bolt guns out to 1000 meters? Is there such a thing as "the best" setup?

[sinister]

Figure out what you need the tool to do.

An AR-10 carbine, HK51, OBR, or SR-25K fits a distinctly different mission requirement than a sniper rifle.

135 and 155 Match Kings (as well as Ball and M993 AP) out of a 16-inch barrel is quite a lot of kinetic energy.

If the shooter wants to use his standard SOPMOD enhancements and doesn't want or need the AK, why use an AK?

[ST911]

With conventional .308 rounds, it is my understanding that it takes an 18" barrel to fully burn the propellant. Shorter than 18" and you increase muzzle flash.

Muzzle flash was significant (think: fireball) from my 16" Ruger Compact with mil ball, import, and some commercial loads until I installed a flash hider. With the flash hider, it is still present but more limited.

With the advent of the Mk 319 Mod 0, where the load is optimized in shorter 16" barrels, compact battle rifles will be much more practical. I'm hoping this trend in caliber development (Mk 319 Mod 0) will spill over to the civilian/LE side with manufacturers. Maybe Hornady will produce a "short barrel" .308 round with 130 gr AMAX for their TAP line?

There are loads coming to the market which will be optimized for short barrels. Right now they are not commercially available.

[dorchester]

I really don't know if a 16 inch .308 FMJ will fragment at even 5 yards, what little I've seen and recall implied something like 2700 fps was needed to fragment an FMJ .308. Muzzle velocity on this barrel is about 2620 fps with German 147 gr surplus ammo, 2550 fps with Portugese and 2700 fps with most commercial 150 gr softpoints. If it's not going to fragment I almost might as well have an AK or VZ-58, unless I use expanding point bullets. That's the conundrum: is a 16 inch .308 giving you what you pay for (in weight, size, muzzle blast, hassle and money)? For now, I'll keep the 20 inch upper on my AR-10, at least until I can find research or test on my own to see if 16 inches will fragment at a reasonable distance downrange. Anybody got the data out there?

[DocGKR]

While a few specific loads do fragment at higher velocities (generally above 2700 fps), the vast majority of 7.62x51 mm FMJ projectiles do NOT fragment at any normal velocities in barrels 26" and less.

Except for certain mission specific tasks, I prefer .308 rifles with 16-20" barrels--this offers excellent terminal performance with the common early upsetting 130-155 gr projectiles, as well as good accuracy with the 168 & 175 gr loads match loads. The recent KAC SR25 EM and LaRue OBR with 16" barrels are superb examples of this type of rifle.

[dorchester]

Interesting data, thanks for that. It appears that for superior lethality the 7.62X51 is best with HP/SP style bullets in most any reasonable barrel length. It makes me wonder if the reputation of the battle rifles for superior power is mostly a penetration of barricades issue rather than wounding mechanisms.