I've noticed some AR barrels have 5R Cut rifling along with some bolt actions. What is the difference between this and say hammer forging? Is this a more accurate rifling? What are some of the benefits? Also how about 6R barrels?

There are no 6R barrels to my knowledge.

Hammer forged barrels are perhaps the lowest quality available. They are meant to be mass produced at the lowest cost per unit. The equipment is very expensive to begin with but costs the least per barrel to finish.

5R barrels have a radius on the rifling that is suppose to reduce stress on the bullet jacket. The very best and most expensive barrels are cut rifled one groove at a time and hand lapped to a fine interior finish. Bartlein, and Krieger would be premium examples.

The other premium barrels would be button rifled like Pac-Nor, Shilen, Douglas, Criterion, Wilson to name a few. The rifling button can be pushed or pulled through the bore, it makes for a smooth interior finish.

Barrels have never been better than what you can buy nowadays. Even hammer forged barrels can shoot very well with match grade ammo.

If you want any AR to shoot well feed it premium chow. Sierra Match King bullets have shot lights out in every AR I fired them through, even rack grade chrome lined barrels. M193 and M855 is crap ammo and rarely shoots under three inch groups at 100 yards even out of the very best barrels on the market. I'm not talking 3 shot groups. 10 rounds of match ammo will usually shoot under 1.5" groups with a scope off sand bags from any good AR-15. Closer to .75" groups from a 1/8 or 1/7 twist stainless steel match grade barrel.

5R rifling is patented.  To do it, the barrel maker must have a license from the patent holder. Barrett (Boots) Obermeyer holds that patent.

5R FAQ

Only a few companies have been granted a license. Interestingly, one of the license holder, Thompson Center, was acquired by Smith and Wesson.  This is why more than a few of S&W M&P 15 better models have barrels sporting 5R rifling.

It is debatable as to how much better 5R rifling is from an accuracy standpoint.  It is but one part of the complex interplay of machining operations that make the difference between a great barrel and a so-so one.  You can't make a poor barrel shoot well by using 5R rifling, and there are great barrel makers that produce superb results without 5R rifling.  But, in combination with high barrel making expertise, then it can be beneficial.

At a minimum, 5R rifling produces less copper fouling and is easier to clean.  When combined with all the skills of a first class barrel maker, it can improve accuracy, increase barrel life, and make an already accurate barrel shoot longer between cleanings.  It is not, however, the most important factor in barrel accuracy.

The M&P 15 barrels are made in house by the subsidiary, Thompson Center, which has a reputation for accuracy and knows what it is doing.  They are 4150 CMV steel and nitride treated.  This combination is still not the same as a stainless match grade barrel, but does produce some very fine shooting, highly accurate tactical type barrels.

5R is the contour and shape of the rifling.  It is independent of the method used to make the rifling.  You can have 5R rifling in single point cut, button broach or cold hammer forged barrels (the three methods of rifling a barrel).

Thanks for the info.

I have an AAC MPW in 5.56 that the barrel is a 5R. Just made me curious.

I did contact AAC and they told me that they manufacture their barrels in house.

You already got a great explanation but FWIW, my 300 Win Mag is a 5r and it is the most accurate gun I own.

Hi,

The basic 5R system has been around for a long time and was invented by W. E. Metford for military and target muzzle/breeech loading rifles in the 1860's see #4 below.  It had an excellent reputation for accuracy.



The only difference between the current ant past pattern is that the current pattern has a more angular profile and is set up for breechloading only systems.

Regards: