4 groove Krieger barrels

[Jgun]

I intend to research this further but this is my first stop. I figure I'll see if the experts over here can shed some light on this. All of my AR's (I think) have 5 groove rifling in their barrels, even though most came from different manufacturers (except two Noveske barrels) I've been looking at M1A barrels from Krieger and others, and notice that many have 4 groove rifling. Now we're talking about gas guns of the same caliber (.308) so I'm trying to find out why they are so popular for the M1A's and not the others. I would think that you'd get better accuracy from a 5 groove barrel than from a 4 groove.

[Robocop1051]

I'm looking for the topic I found online once. It said something about the M1A having a bedded barrel and the other barrels being free float. I don't remeber everything that was in the thread (I think it was on snipershide.com). Something about the M1A barrels designed to be wrenched down into the frame.

I'll keep looking.

Update:

I found a few more items on THR and Snipershide.com. The more I look at the barrel design I see that the M1A was designed for long distance heavy loads using a 1:10 twist and long barrels. Perhaps the 4R design is part of the harmonics of shooting heavy loads with that twist out of a bedded barrel?? A heavy bullet spinning that fast could cause a lot of friction. Especially if it was being forced through numerous rifling grooves. This is just a theory but do you think that, to slow down barrel wear, they reduced the number of rifling grooves as a standard practice? More friction in the barrel increases wear, and is also slows the bullet down a tad. Perhaps they are getting longer shots and a longer barrel life with only 4R when shooting the heavy loads.... Again, just a theory. I have nothing to back up anything I just wrote.

[Jgun]

Thanks for your research on my behalf Robocop. Maybe the point that you made about the M1A not having a freefloat barrel could have something to do with it, but aren't AR's with standard handguards non free float? The set up that I am going with will have a stock that doesn't glass bed the action, it is supposed to be close tolerance machined for a snug fit, with the action bolted in to it. At the mfg's reccommendation I ordered the optional barrel tensioner, which is supposed to improve the accuracy of the non free floated barrel. I'm just guessing here, but was thinking that maybe the reason for the 4 groove barrels is because that is what they used when the guns were first mfg'd in the 50's. Of course, I'm sure that someone has tried out other types of rifling in the past 50 years, and if it really showed a marked improvement in accuracy, everyone would be shooting the other barrel type.  I believe your right about the 4R rifling having less resistance than 5R, and that should equate to lower pressures. I was told by one of my barrel makers that the longer rifling 1/12 vs 1/10 gives slightly lower pressures. I plan to give Krieger a call this week, and if I'm lucky enough to get someone on the phone that's willing to take the time to answer this, I'll post the information I receive.

[weaponizer]

Let me throw another thing in the mix, maybe the rifle being designed to fire full auto had something to do with it, i'm just guessing here, being it was mentioned about the less friction of the 4 grove... <dontknow>

[Jgun]

I spoke to Krieger today, The person I spoke to was very helpful and patient with my questions. What I came away with from the conversation, was that Krieger in their testing had found no advantage or improvement in accuracy between the 5R rifling which was developed by Obermeyer, and their 4R rifling. I think that it's not so much a matter of whether the barrel is 4R or 5R rifled as much as it is a matter how well the rifling is executed. I think that what your paying for from Krieger is their attention to detail more than the particular type of rifling. I'm guessing that, with their reputation as makers of some of the best barrels available, if there was a clear cut measurable accuracy advantage from 5R rifling, they would offer it. That being said, you can get 5R barrels for the M1A/M14 if you want, just not from Krieger. So I guess that when you buy your barrel from a reputable company your more likely to be getting an accurate barrel.