I read somewhere that the 5R designation on the barrel of the 15sport has something to do with an increasing twist rate of the rifling of the barrel. Is this true? What does it actually do to the bullet and what purpose does it serve. I am not sure it is true but I havent seen that designation on any aftermarket barrels I have seen in any catalog. Please help me to understand what I am looking at.
Thank you very much!

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Grover Oathkeeper

Technology - Thompson/Center

thank you Dragon88 and I apologize for an earlier rant on another thread we both posted on!

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Grover Oathkeeper

Basically 5R means the lans and groves are cut at 65 degrees instead of 90 degrees ..in short.

The rifling twist also starts longer then tightens to 1:8.

I have no idea what benefit that little factoid means in the real world. I'm still researching.

I have noticed that a lot of the rifles have that little turned down area for attaching a grenade launcher and the sport doesn't. Does that area set up any weird harmonics in the barrels that don't exist in the sport? I have some long distance hunting rifles and have been learning about harmonics tuning to increase accuracy and saw the area that doesn't exist on the sport and wondered.

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Grover Oathkeeper

means instead of 1:7 and 1:9 its in between to allow for a twist rate that you can successfully fire lighter and heavier loads. Twist rate just allows for the stabilization of the bullet.

Thanks!

Do you know the effect of the rifling starting out loose then tightening to 1:8 towards the end of the barrel v.s. a barrel that has 1:8 rifling consistent through the entire length of the barrel?

No the mounting cut will not act like an hourglassed barrel.

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Hold my beer and watch this!

This is called Progressive Rifling. To explain this we need to start at the instant when the cartridge is fired, and the bullet surges forward from the case. The bullet goes from stationary to 3000+ fps, and begins rotating thousands of revolutions per second, in a fraction of a second under extreme pressure and heat. This is very hard on the bullet, and particularly the jacket. When the bullet travels through the leade and hits the bore, the rifling cuts into the jacket and upsets it. The jacket on a bullet, particularly a match grade bullet, is very important. The more it is disturbed during firing, the less consistent your groupings will be.

A rifle bore with regular rifling will cut into the jacket immediately when the bullet hits the bore. In a barrel with Progressive Rifling, the twist rate starts slower. This gives a more gentle start to the bullet as it begins its trip through the bore, and slowly buildings it up to the final twist rate.

Shooting standard FMJ bullets at close ranges, the benefits of Progressive Rifling will never be seen. At long ranges, with match grade bullets, you will get noticeably improved consistency from a 5R progressive barrel. Match grade bullets have relatively thin jackets, and benefit more from Progressive Rifling than regular FMJ.

Dragon88,

Thank you for taking the time to answer my questions. I know it can get irritatingly repetitive answering novice rifle questions, and I really appreciate it.

I wish there was a button I could push & an ice cold beer would appear next to your keyboard.

Cheers!

JaPes...I feel the same way about the beer button or if my dog would just bring them some...Thank You Gentlemen for answering my questions..My Local FFL Dealer is closed on Mondays but my Rifle ships tomorrow...I would love to have it before the end of the week...I will post pics as soon as it gets here..Everything is supposed to ship at the same time...3 mags, 1 M&P 15 Sport, 1 Ultradot Pan-a-v Holographic sight and 100 rounds of ammo...I am like the kid wanting his Red Ryder BB Gun...I am so excited...Thank Everyone Very Much!!!

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Grover Oathkeeper

No problem sir, thats what we are here for! Enjoy your rifle! FWIW Cheaperthandirt has BRV ammo (once fired) on sale 500 rounds for about 150. It shoots great through all our AR's.