16” vs 18 inch 6.5 cm gas gun velocity difference?

happyfast79

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  • Mar 27, 2013
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    I’m looking at rebarreling a large pattern AR to 6.5 creedmoor, Use of the rifle will be a all around mid range 100-700 yard gun with some 1000 yard stuff everyonce in a blue moon and may take it hunting here and there. I would like to keep it light as possible while keeping it decently accurate with smaller shot strings.

    My budget isn’t great so I’m looking at faxon big gunner barrels and am debating between 16” and 18 inch. Anyone have any real wold velocity numbers out of a gas gun between the two lengths with heavier (130-150) grain bullets? Thanks .
     

    drewthebrave

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    Aug 20, 2019
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    I'd go for the 18" if only to get the Rifle length gas system. 6.5 Creedmoor has a longer pressure curve than .308 (similar pressure, smaller diameter), so there's a lot more pressure at the same port location compared to .308. In general, the farther away from the chamber the gas port is located, the smoother your action will cycle. A well made barrel with a properly sized gas port can mitigate the issue, but most rack grade barrels (like this Faxon) tend to be over-gassed for reliability purposes. An adjustable gas block or gas key on your carrier can help tame the system's cyclic rate -- but the longer gas system allows more volume for the gas to expand before cycling the action, resulting in a less violent cyclic rate. I would be very hesitant to use a midlength gas system with 6.5 Creedmoor.

    For what it's worth, my 18" Craddock Precision 6.5 Creedmoor barrel has a Rifle +1" Extended gas system, and it runs like an absolute champ, with recoil impulse not much worse than my 18" midlength Grendel.
     
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    Snuby642

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  • Feb 11, 2017
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    I have not tested 6.5cm but with loads I got to test it was 23-24 fps per inch on some 223's and 32 fps per inch on 308.

    It will change with ammo and barrels. It seemed faster rounds lost less per inch than slower rounds.

    This was done with a cheap chrono with mixed factory and handloads.

    FWIW
     

    vinniedelpino

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  • Sep 27, 2020
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    I'm already running mild loads in an attempt to get four firings out of my brass. If I were running a 16" I'd probably be in grendel bolt gun territory.

    What am I missing?
     

    happyfast79

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  • Mar 27, 2013
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    I'd go for the 18" if only to get the Rifle length gas system. 6.5 Creedmoor has a longer pressure curve than .308 (similar pressure, smaller diameter), so there's a lot more pressure at the same port location compared to .308. In general, the farther away from the chamber the gas port is located, the smoother your action will cycle. A well made barrel with a properly sized gas port can mitigate the issue, but most rack grade barrels (like this Faxon) tend to be over-gassed for reliability purposes. An adjustable gas block or gas key on your carrier can help tame the system's cyclic rate -- but the longer gas system allows more volume for the gas to expand before cycling the action, resulting in a less violent cyclic rate. I would be very hesitant to use a midlength gas system with 6.5 Creedmoor.

    For what it's worth, my 18" Craddock Precision 6.5 Creedmoor barrel has a Rifle +1" Extended gas system, and it runs like an absolute champ, with recoil impulse not much worse than my 18" midlength Grendel.

    Thank you for the info, I will probably wait for a Black Friday sale and try to pick up a 18 inch. From what I was seeing it doesn’t seem to get any significant benefit in velocity untill you get over 20 inch. So I probably wouldn’t gain much from going to a 20 inch bs the 18 inch?
     

    TonyTheTiger

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    Feb 14, 2017
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    From what I was seeing it doesn’t seem to get any significant benefit in velocity untill you get over 20 inch. So I probably wouldn’t gain much from going to a 20 inch bs the 18 inch?
    Just figure 20-30fps per inch. Only you can decide where to draw the line on the length/weight to velocity ratio.
     

    mheimer_45

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    May 19, 2013
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    My experience with faxon barrels is they make better boat anchors then rifle barrels. I was dumb enough to try them a second time and both have been replaced. But that’s in .223.
     

    happyfast79

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  • Mar 27, 2013
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    My experience with faxon barrels is they make better boat anchors then rifle barrels. I was dumb enough to try them a second time and both have been replaced. But that’s in .223.
    Any recommendations on a lighter barrel that is better quality at less than $300?
     

    TonyTheTiger

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    happyfast79

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  • Mar 27, 2013
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    Stupid amounts of back pressure if you are running a midlength gas...... as others have said...+1 or +2 gas....prolly a hp bolt as you will prolly be smashing primers and have really violent cycling
    Ok thanks for the info, the barrels on the 2 rifles that I bought are 18 inch midlength 308 and I have not shot them yet. I was going to keep the 308 that shot the best and rebarrel the other one to 6.5 cm
     

    mheimer_45

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    May 19, 2013
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    Also I should throw out there that the rifle has a adjustable gas block on it

    Then your fine. All my rifles (as in double digits) have adjustable blocks. I have everything from +2 to carbine. As long as the gas system is tuned your fine. No HP bolts but I do run the standard SCS in everything.
     

    bfoosh006

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  • Jun 13, 2007
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    IMHO... spend more money for a better barrel. After looking around I see that is easier said than done, Lol... not much in stock 6.5CM wise..

    But given the current cost of all things like ammo, ( $1.50 / rd and fast going to $2.50+ / rd for "decent" long range ammo ) and powder / primers/ brass.... a better barrel is a bargain dollar wise.
    Especially given the decent long range ammo current price ... about 50 bucks per 20rds.

    A better barrel offers a greater chance at being a great shooter.... with a larger spectrum of ammo choices. ( Less searching for a round it likes )

    Your Faxon choice could be very good... but , frankly, I rarely see a lot of threads ranting about them.

    I do have a few Faxon 5.56 barrels and they are good enough for their intended use... but I was using your same criteria, a very light weight barrel for those builds.

    Anyway... I would sign up for "in stock" notifications and save up for a better barrel. Or have one turned up for you.

    I'd bet you'd enjoy a slightly heavier , more accurate barrel in the long run. Especially at the longer ranges.

    YMMV.
     
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