Author Topic: testing procedure  (Read 4186 times)

The other DWS

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testing procedure
« on: February 04, 2010, 11:40:10 PM »
I know I'm getting the cart WAY before the horse here (I'm still waiting for my barrel to get here) but the topic came up in a conversation and I thought I'd ask the experts here.

When you are testing a long rifle for accuracy how do you benchrest it?

I do a lot of ASSRA shooting and our events include both bench rest and offhand matches for out single shots.   We have guys who make a religion about bench position and barrel harmonics etc etc.
I was wondering with such a long barrel that is pinned into the stock, how do you rest the rifle to get a good reliable test of its accuracy for load development?

Daryl

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Re: testing procedure
« Reply #1 on: February 04, 2010, 11:50:06 PM »
I grasp the forend just as if I was shooting offhand - at the entry pipe, finger down the pipe and rest the back of that hand on the adjustable rest's sand bag - no rear bag is used.  Both elbows are on the table.  This give me the same point of impact off the bags as shooting offhand.
If purely testing loads, I might for-go holding the gun and assume a standard BR hold on the gun, using a rar bag and resting the forend at the entry pipe letting the gun recoil and bounce as it wants.  This can shave 1/2" off group sizes compared to the hold noted above, but can give up to an inch and generally more change in the point of impact.

beleg2

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Re: testing procedure
« Reply #2 on: February 05, 2010, 03:06:53 AM »
HI TODWS!

We meet again. I remember you from TT.

Martin

The other DWS

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Re: testing procedure
« Reply #3 on: February 05, 2010, 03:45:59 AM »
yeah,  I have several different interest areas,  all focused on "traditional" ;D

Offline volatpluvia

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Re: testing procedure
« Reply #4 on: February 05, 2010, 06:40:04 AM »
The longrifle I made for myself in 1991 would not shoot a group off sandbags.  It had to be held as if I was shooting offhand and left wrist on the sandbag and elbows on the table.  It had a Getz barrel 'C' weight in .54 cal.  If I rested the barrel forward of the entry pipe on the bag it shot a round pattern.  If I rested the barrel on the entry pipe it shot a narrow vertically dispersed string.  Getz said it should not be so.  But it was.  So I shot it mostly offhand and it shot wonderfully that way at the long bears on the steel sillouette range.
volatpluvia
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Offline Rich

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Re: testing procedure
« Reply #5 on: February 05, 2010, 07:37:37 AM »
I rest the forestock on sandbags, the butt of the rifle where it will be if shooting off hand, and my left hand holds the butt forward of the toe.

Offline Darkhorse

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Re: testing procedure
« Reply #6 on: February 05, 2010, 08:04:11 PM »
I rest a longrifle same as any other gun I shoot off the bench. Sandbags front and rear.
I expect my rifles to shoot well no matter how it's rested or what it's held against (such as a tree) if not the rifles useless to me. And I will try to find out why and fix it.
American horses of Arabian descent.

The other DWS

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Re: testing procedure
« Reply #7 on: February 05, 2010, 11:23:46 PM »
Other than having a solid and repeatable rest from shot to shot the big difference is in the barrel harmonics.  the main reason the unlimited benchresters who are competing on sheer group size use short very stiff barrels.  I'll bet that with our longrifle barrels we get some really dramatic harmonic vibrations going.  That would make them really sensitive to the front rest position/method.  Shooting off the "rested" hand would dampen them some.

I'm speculating too, that with a relatively large diameter ball and a long relatively thin walled tube torque on the barrel might even be a factor--giving a donut pattern if the front rest was too hard and in the wrong harmonic node position.  Heavier stiffer barrels and/or smaller diameter projectiles might not be as effected

Mike R

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Re: testing procedure
« Reply #8 on: February 06, 2010, 01:04:33 AM »
I am trying to understand why placing a sandbag on bench under the rifle where your hand would normally be offhand is any different harmonically from the offhand position?   

Daryl

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Re: testing procedure
« Reply #9 on: February 06, 2010, 04:43:43 AM »
As long as you grip the gun, it's the same Mike. Ungripped, the gun, pivoting on your shoulder on recoil, jumps higher & faster therefore doesn't shoot identically to offhand, or when resting the back of your hand on the bag as you grip the forend as I try to do.  I get over an inch difference in two rifles.  Gripping is the only way I shoot the bigger guns, due to recoil.  I don't want the gun's forestock slamming back onto the bag and perhaps breaking the stock or possibly bending the barrel.
Stock configuration will also change recoil patterns. The straighter the stock, the less difference in impact due to holding changes.

The other DWS

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Re: testing procedure
« Reply #10 on: February 06, 2010, 06:38:55 AM »
Remember too that recoil starts acting immediately.  With a longer barrel that means that recoil will be whipping the barrel out of your sight alignment long before the ball exits the muzzle--relatively speaking of course.  Now barrel mass might have an influence but not all that much.  And of course heavier charges and larger diameter/heavier balls will create more recoil, especially in a barrel with the same outside diameter.  There's a real good reason why target shooters use lighter loads.

Letting a rifle "free recoil" can help with consistency and most of the high performance bench resters do just that.  However I suspect that trying to shoot a M/L long rifle out of a conventional bench rest setup would be challenging to say the very least unless one were to design some really specialized bags.   I have tried it using fullblown schuetzen offhand stocks, which have a lot more in common with longrifle stocks than they do specialized benchrest stocks, and it is a real exercise in frustration.

And here I decided to get back into muzzle loading thinking it'd be so much simpler and a nice break from the schuetzen game----what was I thinking :)

maybe this spring when I get my flintlock built I can do some experimenting with my bench setup but I strongly suspect that it'll require some pretty solid controlled recoil management to get decent test results---if I can figure out a good location, barrel harmonics-wise for the front rest.

Daryl

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Re: testing procedure
« Reply #11 on: February 06, 2010, 08:37:20 PM »
Yes - where you rest the gun's forend or barrel can make a big difference in group size and location as well.