stuff on barrels versus accuracy


September 19, 2009, 09:12 PM
I have read with bolt actions the best practice is to bed the receiver but allow the barrel to free float. With semi automatic piston rifles this is not possible because the barrel is connected to a gas tube which is connected to the receiver. Plus if it is a military rifle more often or not there is a bayonet lug and maybe a muzzle brake. To my understanding the concept of free floating a barrel doesn't work at all. I have a semi auto, FN49, which is proven capable of sub MOA accuracy but I have read reports (complaints) of shot stringing. Wouldn't accuracy in a design that already constrains free floating barrels benefit from the opposite, bedding as much of the barrel as possible? I guess trying to get the effect of a thicker bull barrel.

Remember I am asking for illumination senseis. Along the same line is harmonics. This is part of a conversation I was having with someone about muzzle brakes.


Reloading will require a different load due to changed harmonics. If the muzzle brake is tightened too much it will cause some torque to the bore and will diminish the rifle's accuracy. It does not take a lot of torque to cause problems. The answer is to only snug the brake gently, yet firmly to the threads, not going over 15 - 20 lbs of torque.


I can't really see if you over tighten something threaded a bit everything is going to go to hell in a hand basket. I mean the barrel is thick and whatever torqued force would be on the outside just at the thread line.

I can see how adding some weight to the end of the barrel can change dynamics.

If I could get rid of the bayonet lug would it have a positive effect on accuracy?

Bottom line with a barrel with a piston attached would it be a good idea to have as little stuff hanging on it as possible and to bed more of the barrel?

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September 19, 2009, 09:36 PM
Free floating the bbl will reduce the effect of the stock expanding and contracting (all stocks do it to some degree) on the barrel and therefore a change in POI. Also when not free floated the stock will "push" unevenly (often affecting windage as well as elevation) as the barrel heats up. This is not to say that a properly bedded barrel cannot be more accurate than one that is free floated...but this is typically the case. IMO the best course of action is to free float the barrel first (even an auto that cannot be fully free floated), and if it doesn't shoot as accurately as you would like (and you don't want to invest in a new barrel) then bed the barrel with mild upward pressure in a good synthetic stock (this will minimize the uneven expansion and will not swell in humid conditions).

Over-torquing the muzzle accessory (whatever it may be) should be avoided because it can cause stresses in the bbl and can actually cause a constriction in the bore reducing accuracy if tightened very excessively.

I don't believe that the additional weight of a muzzle device will affect accuracy that much, but some devices can adversely affect accuracy due to their design as some impede the natural flow of the gas exiting the barrel. DTA had problems with a flash hider and therefore stopped using them on their rifles because it caused the rifle to shoot above much above I don't really know. OTOH a suppressor did not have the same effect on the same rifle. Also I don't believe that the bayonet lug will make a big difference either.

Bottom line if it is a MBR or other semi auto you probably won't notice much of a difference with any of these "upgrades"...unless of course it is an accurized rifle (like a blueprinted national match M1A). :)

September 20, 2009, 10:15 AM
methinx bbl vibration length is the variable that effects POI when brakes are added. it could explain a brake reducing accuracy, and a suppressor aiding it on the same rifle.

in way of anectdotal proof, consider the "B.O.S.S." system. all it allowed was the ability to fine tune the barrel vibration length.


September 20, 2009, 10:54 AM
Interesting gunnie, I'll buy that, barrel harmonics can make a big difference. :)

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