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Thread: Minimum barrel length and twist for XM193

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    Minimum barrel length and twist for XM193

    What is the minimum barrel length needed to get enough velocity out of XM193 or any 55 gr 5.56 or .223 load to make it effective? I thought it was 16" but have heard conflicting opinions. Oh and is a 1/7 suitable or doable for the lighter 55 gr bullet loads? I know 1:9 is preferred. Thanks

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    IIRC, 55gr fragments at 2700fps and above.

    DocGKR would be the one to ask these type of questions, he's done a lot of ballistics testing.
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

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    Thanks gotM4 for your always prompt help. Don't think I'm gonna get 2700 fps out of a 14.5 barrel with XM193. Oh I don't the good Doc care's for me. Though I do adhere to his advice.

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    Actually, you can get 2700fps from a 11.5" barrel with XM193.

    Here are the fragmentation ranges for various barrel lengths for XM193:

    20" - 190m
    16" - 140m
    14.5" - 95m
    11.5" - 40m

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    Quote Originally Posted by User Name View Post
    Thanks gotM4 for your always prompt help. Don't think I'm gonna get 2700 fps out of a 14.5 barrel with XM193. Oh I don't the good Doc care's for me. Though I do adhere to his advice.
    Average muzzle velocity for XM193 is about 3050fps out of a 14.5" barrel. So it's probably good to about 75-100yds for reliable fragmentation. Personally I would use 68gr HP Hornady or 69gr HP Sierra for 'carry' ammo in an under than 16" barrel with 1x9 twist (if you can get the FBIs rifle ammo even better, IIRC their load is the Federal 64gr). For less than 16" 1x7 twist barrels my 'carry' ammo is 75gr Hornady TAP or 77gr Black Hills red box.

    A FBI friend of mine out of Quantico says that their load works better than the 75gr and 77gr rounds. He says the 75gr and 77gr outperforms their 64gr loads in his words only in the 'naked man theory' meaning no barriers. I'm sure his opinion is a little biased as he did a lot of testing it and had something to do with it's adoption.

    I've moved this thread to the Ammo and Reloading thread.
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

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    Quote Originally Posted by User Name View Post
    Oh and is a 1/7 suitable or doable for the lighter 55 gr bullet loads? I know 1:9 is preferred. Thanks
    The ideal rifling rate for .224" 55 grain bullets is 1 turn in 12" (1/12). 1/14 will work with 55s, in all but very cold (very dense) air.

    Anything faster than that will also work for 55 gr FMJ and HP match bullets and will not decrease accuracy to any appreciable degree so long as the rifling rate isn't ridiculously fast (1/5 or more). Thin-jacketed varmint bullets tend to disintegrate in flight due to excessive centripetal forces if shot at very high speed through faster than optimal rifling.

    I have personally shot lots of 50 to 55 grain FMJ and HP match bullets in short range HP competition through 1/8 and 1/7 barrels with excellent accuracy and no ill effects.

    1/9 is ideal for bullets 55 to 70 grains. 1/9 may stabilize 75 grain projectiles if the barrel is long enough to attain at least 2700 fps (a 20" barrel will do). Less MV than that and the rotational velocity imparted by the rifling will lead to instability in flight. If you want to use shorter than 20" barrel, 1/8 is the minimum rate for 75 gr and heavier bullets.

    Contrary to errornet belief, the 1/7 rate was not adopted by the .mil to stabilize the M855 62 grain bullet, but to stabilize the much loinger M856 tracer bullet. Length, not weight, is the key factor in deciding how much rotational velocity is needed to stabilize a bullet.

    More than you wanted to know, maybe, but that's what keeps us a cut above other sites on the net.

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    Thank you very much guys. Hopefully one day I will have enough AR info stored in the old brain bucket to answer a future newbies questions one of these days and give you guys some well deserved rest! I asked because I purchased an Noveske N4 and after handling the 14.5 + perm attached flash surpressor my 16" 6920 felt clumsy. So who should I send my 6920 upper to to get it rebarrelled with a 14.5 barrel and a 1.5 f/s? Grant? Or should I just swap uppers? Which I would do but I got married last week and went from comfortable to poor in five minutes. So money is an issue.

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    Quote Originally Posted by gotm4 View Post
    For less than 16" 1x7 twist barrels my 'carry' ammo is 75gr Hornady TAP or 77gr Black Hills red box.
    Same for me (load wise). I just figured it would be good to have a rifle set up that could shoot 55gr loads (if necessary) as they are so commonly available.

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    Quote Originally Posted by User Name View Post
    Same for me (load wise). I just figured it would be good to have a rifle set up that could shoot 55gr loads (if necessary) as they are so commonly available.
    A rifle that is capable of stabilizing heavy (75 - 77 grain) .224" bullets is automatically set up to stabilize lighter ones. You absolutely do not need another rifle "set up" differently.

    I am truly mystified as to why this subject continues to cause so much confusion and misunderstanding.

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    I really only have 55gr. FMJ, I use .223 for plinking, and keep around 1100 rnds of assorted 5.56 pressure stuff on hand for whatever needs it. Most of the 5.56 stuff I have is Guat, its my "go to" ammo, despite it being foreign made, I think the 5.56 pressure is a bigger advantage than using domestic ammo. Not to go off topic, but since most of you guys prefer "rarer" loadings like 75+77gr. not to mention various loadings in the 60's, is there anything particularly wrong with 55gr FMJ?

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