AR build question

[saggs]

Exhibit C:

Higher stresses and lubrication burn off from direct gas impingement versus what this paper calls an "external piston".

http://www.ewp.rpi.edu/hartford/~ernesto/SPR/LeBlanc-FinalReport.pdf

Sure, if you never, ever, ever, ever, clean your rifle then a piston 'might' be more reliable.  But I think that 50+ years of military service and millions of DI rifles in the hands of civilian sportsmen have proved that DI rifles are plenty reliable if given proper care.

The only disadvantage I've ever seen to DI rifles is the gas in the face if you're shooting suppressed.  Some people don't like the spring noise I guess, but that's never bothered me.

[Gman]

What Saggs said in the first post - the 300 BO round is not much different than a 762x39 round IMO, and the only reason I have one is to have a round that is versatile for using both suppressed and unsuppresed fire.  It's really the best round out there for the AR platform now which allows you to do both well.  If you don't care to suppress a rifle, you're not taking advantage of 1/2 of the benefit of 300BO, so 300BO is out on that score for the OP IMO.  One thing to take into consideration is the magazine issue - 300BO you can use PMag and standard metal mags.  6.5G, not so much, so you'll need to get a separate stock of magazines for just one rifle, but considering the $ of mags, not a huge deal, but one to be brought up at least.



Travis Haley ran a 300BO rifle for an entire year on every course he taught and attended, if you check out his writings and videos on it, he probably has the most thorough explanations and valid opinions regarding all aspects of the practical uses of the 300BO round. 

Considering the ranges brought up in the OP, the 6.5 is a better choice, as at mid to longer ranges it is typically superior to the 300BO, and the 6.8 as well.  Plus with a 556 rifle with a 1/8 or 1/7 twist barrel set up for heavier bullets like the Mk262 or other 77gr/etc rounds, Maverick should have a pretty hard hitting rifle in that one for mid range already, so a 6.5 will give performance that'll push out a little further compared to that, where as a 300BO won't.

[DaveBB]

Designed fragmentation is pretty specific to small light calibers like .223 because they are just to light to reliably expand.   And because the military is not allowed to use expanding bullets, they make fragmenting FMJ rounds like M855 and M193.  I was thinking in terms of larger heavier bullets since we where discussing .300 AAC and 6.5 Grendel.

It's apples and oranges.  Nobody hunts anything bigger then varmints with lightweight .223 FMJs.  Since we where talking about the 6.5 Grendel, and since I built mine for medium to large game hunting I was talking about hunting bullets.

We're both right, just talking about different things.

A post-Vietnam War study found that it only took two 5.56mm rounds to kill a person with a 90%+ probability.  This is how the 3 round burst came about.  If 2 rounds had 90% probability of killing someone, 3 rounds would almost guarantee it.

Also, this pretty specific to the U.S. 5.56mm round.  It enters the body, yaws 90 degrees, flattens out, undergoes around 70 g's, and then catastrophically fragments.  It takes about 7 inches to do this, so it definitely penetrates first.  Interestingly, the Soviets were not able to replicate this type of wounding with their AK-74. 

[Vulcan]

Suppressed 300BLK is on my wishlist, maybe one of the light scout setups.

[FX1]

A suppressed 45 is the wtg if your going that route. I sold my 300blk because ammo was three times the cost of my ak's and my carbine 45 suppressed is more deadly with modern hollow points like federals Hydro shock and HST. The 6.8 is a better platform that inst a 223/762.39. The 300 is going to stay around and i do see it becoming more popular once people get out into the woods and find out that the 223 is very light for north american game.

[Ripsnort]

A post-Vietnam War study found that it only took two 5.56mm rounds to kill a person with a 90%+ probability.  This is how the 3 round burst came about.  If 2 rounds had 90% probability of killing someone, 3 rounds would almost guarantee it.

Also, this pretty specific to the U.S. 5.56mm round.  It enters the body, yaws 90 degrees, flattens out, undergoes around 70 g's, and then catastrophically fragments.  It takes about 7 inches to do this, so it definitely penetrates first.  Interestingly, the Soviets were not able to replicate this type of wounding with their AK-74.

This was true in pre-1968 AR's that had the 1/14 twist. The complaint was...not very accurate.
Post 1968 they went to a 1/12 twist, stablized the bullet, more accurate. The soldiers complained that the enemy didn't go down immediately like they did with the unstable 1/14 flight travel of a 5.56mm.

At least this is what I've read in books.

[DaveBB]

This was true in pre-1968 AR's that had the 1/14 twist. The complaint was...not very accurate.
Post 1968 they went to a 1/12 twist, stablized the bullet, more accurate. The soldiers complained that the enemy didn't go down immediately like they did with the unstable 1/14 flight travel of a 5.56mm.

At least this is what I've read in books.

 

While you are correct in a change of the rifle twist, that did not have any effect on bullet fragmentation.  The fragmentation is due to the bullets center of mass being so far rearward.  It yaws violently once entering the human body and breaks up.  Still does to this day with enough velocity.

[Ripsnort]

I would argue it does not yaw 90 degrees when entering the body. At least with a 1/7 -- 1/12 twist rate. Maybe if it hits a rib, yes.
Just youtube ballistic gel tests. You'll see in a nice straight line, with normal expansion of the gel due to cavitation (normal for every high speed bullet) but the tests I've seen, with none-HP 62 gr or 55 gr bullets, most retain their weight with very little fragmentation UNLESS it hits a bone.