Barrel Harmonics

[Shynloco]

Mornin Everybody,
Bare with me for a moment while I ramble on. Had kind of a sleep interrupted night and couldn't get back under, so I got up and tuned in Accurate Shooters to see if there were any new threads of interest. And I came across one in which the poster had an issue of even getting on paper at 100 yds when he changed from a silencer to a muzzle break. That thread prompted my little brain to remember that I had been on a phone just yesterday with a friend back east who'd recently competed in a competition at range near Madison, WI and told me how he'd been extremely fortunate and had been presented with an (award) new Kelbly 7mm Action after the shoot was completed. Knowing his age and potential frailties, I asked him if he was going to put a suppressor on it. He then reminded me you cannot use those types of attachments in competition shooting,

Back to my "whomever moment" that the lights came on in my pee and sleepless brain. I remembered the thread of changing attachments on a rifle barrel, not being able to use attachments in competition and the fact that when I competed in Rimfire, I had a Harrel Tuner on my Anschutz that enabled me to "fine tune" the best ammo my Annie liked.

FINALLY the question (sorry for the memory lane number)... just how much is a barrel's harmonic's affected when we attach things like suppressors or silencers??? And consider most of us use Match Grade barrels and we highly recommend free floating our barrels and even actions, so the barrel isn't dampened by making contact with the stocks we use. I mean how many MOA's or how much is the accuracy of our rifle changed at 100yds. 500yds or even 1K? Thx!

Alex

P.S. Sorry for the long winded BS!

 

[T-shooter]

Putting a stainless brake on my .308, which has a thin barrel, didn't change the POI more than a click or so a mil/mil scope. The thing I noticed was an instant improvement in accuracy, even with different loads. Whatever it did to the harmonics was a plus. And the barrel channel is opened up enough I probably have 1/8" clearance all the way around. Was it the effects of the brake or the added weight? Most likely the weight.

 

[mikegaiz]

I have done a lot of load development for competitive shooting shooting and believe any change to weight, length, or additions (brakes, tuners) will definitely change barrel harmonics, which directly affects grouping.

 

[carlsbad]

OK. One of my favorite Einstein quotes is: "Everything should be made as simple as possible, but no simpler". I will admit that the following discussion may be a bit too simple.

A large portion of load development is finding a node where the barrel harmonics are relatively stable and small changes result in minimal changes in POI. The natural frequency of a cantilever beam (barrel) varies with mass and length. So changing either will change your frequency. Now comes a simplification that may not be universally accepted: a large portion of load development is finding a muzzle velocity that matches the natural frequency of the barrel--the bullet exits the muzzle just as it stops moving to vibrate in the other direction. The worst load would be the load causing the bullet to exit the barrel when it is moving with maximum velocity. If you look at a sine wave, the maximum velocity is as the curve crosses zero and the minimum velocity is as the function peaks and moves almost horizontally for a while before reversing direction.

Thus, if you do anything that changes your natural frequency, it will affect your load development. A load that works well with a muzzle brake may not work well at all without it.

So I'm trying a new approach of "load development" next week. Last night I put an Ezell tuner on my barrel. I will take a load I like and change the natural frequency of the barrel until it matches the muzzle velocity.

Just to be clear, we are not talking about significant changes in POI, we are talking group size.

--Jerry

 

[memilanuk]

The one personal example I can recall specifics on... had a custom rifle - Stiller TAC30 action, Jewell trigger, AICS chassis, NF scope. Original 25.5" Rock Creek fluted heavy Palma barrel in .308 Win, and a muzzle brake (don't recall the model). That gun pretty much didn't care if it was braked or not - shot stupid little groups. Maybe not BR level, but respectable by any other standards.

Same gun, but with a 28" Lilja fluted medium Palma in 6-6.5x47L, Badger Ordnance Thruster brake. It too would shoot stupid little groups - regularly in the 0.3-0.4" range for 5 shots @ 100yds - with the brake on. I got the bright idea to take the brake off and give it a spin as an F-Open gun. Literally took the brake off at the range, and the next groups were 0.75-0.9". WTF?!? Put the brake back on... next group was 0.3", if that. Didn't really feel like working up a different load for the times I wanted to run un-braked, so that was the end of that particular experiment.

 

[Northridge]

Alex, it is my finding that a barrels given tune or harmonic alignment will all ways be affected when you change what is hanging off the end of barrel just a matter of how much, sometimes slight and sometimes dramatic. more often then not with a tune adjustment you can pick back up where you left off. biggest thing you may find different is how it may want to shape a group at a given distance or how wide or narrow a tune window may be.

Shawn williams

 

[chop house]

i don't know about suppressors, but i do know what happens to my sporter profile rifles when i attach the bayo from a magnetospeed. It seems the lighter the bbl the more the poi changes away from the mounting orientation. AND with a few of these rifles i have had groups shrink with the bayo installed... A couple of instances i was not able to get the same precision with the bayo removed even adjusting the load.

I suspect the better built your rifle is the less of this effect you will observe. I say that because on my better rifles the groups almost always open up a bit

 

[CharlieNC]

If you change whats hanging on the end of the barrel I don't think you can properly compare group size without tuning the load for each situation. So how does one directly measure harmonics? I'm working on an approach that may be useful.

 

[Depart123]

If you change whats hanging on the end of the barrel I don't think you can properly compare group size without tuning the load for each situation. So how does one directly measure harmonics? I'm working on an approach that may be useful.

You can't "measure" harmonics, you can only measure change. If you shoot 20 rounds or so at the same POA but use slightly different loads or seating depth, you will see your barrels natural harmonic pattern appear on the paper. It is usually eliptical and typically runs from somewhere around 2 o'clock to 8 o'clock. Although I did have one rifle that was almost entirely vertical until I hit a certain pressure level, then it would sling them sideways. It did this with multiple bullets and powders. I replaced that barrel with a Bartlein and the problem went away.