Keith -- Flexible stock

[Charles E]

Sounds like an interesting project. I suppose you've seen Varmint Al's modeling of a .22 being fired on the Rimfire Acuracy site?

http://www.rimfireaccuracy.com/Forums/showthread.php/1639-Technical-Series-2-Stocks

I also don't know what correspondence you've had with Greg, but he's probably a good source, as he's seen some of the things tried on 1K Light Guns, where there are no stock rules.

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One thing obvious from the RA thread is the rimfire guys have not had any 1K BR experience, as much of what Stiller questions has at least been tried. As with so much in benchrest, tried, with no firm conclusions. Current RF thinking seems to be that a very stiff stock and bedding are quite important. As they don't seem to have many stock rules, I'm surprised they haven't tried just the opposite. When something is critical, oft times a good solution is to just eliminate it.

 

[mks]

Charles,
Thanks for the reference to Al's post over there. I don't get over there much.

His plot of forces on the rests confirms to me that there is potential for tuning with the stock. For a 22rf or other subsonic bullet, there is so much happening before bullet exit, and happening so quickly compared to the variation in bullet exit times, that it is no wonder why tuning is so inconsistent, working one day, but not the next, or even one target, but not the next. What would help is to slow down these responses so that the first large wave happens during bullet exit, and to broaden the response so that exact timing (tuning) is not so critical.

The force on the rear rest, which probably at least roughly corresponds to rotation of the rear of the receiver downward, peaks at about 7 milliseconds, far too soon to help with tuning bullet exit, which occurs at 26 milliseconds. With a 3.6 times more flexible stock (more weight will help, too), the frequency of the response will be decreased so that the peak moves rightward to about 25 milliseconds and gets wider. We want the peak in receiver rotation to happen before bullet exit, so that the wave of motion has time to propagate forward to the muzzle. 1 millisecond is probably in the ball park for the right delay.

One never knows for sure until it's tried, though, so that's why I said the stock I am building is a stab in the dark.

I've been meaning to give Greg a call. Work gets in the way.

Cheers,
Keith

 

[Charles E]

Is Varmit Al's model dependent on a number of variables? Such as where the forearm sits on the rest (how much weight "at" the muzzle)?

I notice the first forearm and second butt peak roughly correspond, as do the 2nd and 3rd. But the next forearm (front rest) peak comes at a rear rest dip. Now, you could do something to move the rear rest peak, but if changing the position of the rifle in the rests also changes things substantially, it is going to be very hard to predict what will work. Trial and error only, I suspect.