The situation described here may be unique to one rifle or it may be typical.  I'd like to determine which and perhaps discover a "fix."

I have a CPA 44 1/2 chambered in .32-40.  The buttstock is attached with CPA's optional "through bolt."  The buttstock was fitted and bedded by CPA.

All screws on the rifle are properly tightened. 

The mainspring has very little pre-load.  It takes very little effort to pull the hammer into full-cock.   

The front rest is a Fudd.  The rear rest is a Protektor donut bag.  Both the front and rear bags are firm but not overly filled.

Here's the situation:

First, push the rifle into battery with the hammer in half-cock.  Adjust the front rest to place the crosshairs at the chosen POA.

Then, pull the hammer into full-cock. Recheck the sight settings and discover that the POA has moved up approximately 3/4" at 200 yards.

Obviously, tensioning the mainspring imposes sufficient stress on some part of the rifle to physically move the POA.  It follows then that that stress is removed when the hammer is released.

Thr result is that the POA constantly changes from the time the hammer is released until the hammer strikes the firing pin.  This cannot be good for accuracy and this rifle has had a real problem with intermittent vertical stringing ever since I first took delivery. 

So, have any of you with similarly constructed CPA rifle noticed a change in POA from half-cock to full-cock?

JackHughs

   

ron wrote on Aug 29^th, 2013 at 11:18am:


Hi Ron,

Yes.  I do the same.  However, that process doesn't cure the underlying problem.

If cocking the hammer (tensioning the mainspring) causes the POA to move, then releasing the hammer causes the POA to move in the opposite direction.  So, from the time the trigger is released until the hammer strikes the firing pin, the POA is constantly changing.

If the change in POA were perfectly consistent from shot-to-shot, there would be no problem with vertical stringing.  However, I don't believe the POA change caused by first cocking and then releasing the hammer is consistent from shot-to-shot.  The mainspring tension-related POA change is a variable and a very bad variable at that.

My intent in this thread is to first find out if other CPA owners observe the same POA change and then to try to figure out how to minimize the condition.  I don't think it can be completely eliminated.

JackHughs 

frnkeore wrote on Aug 29^th, 2013 at 12:34pm:


Hi Frank,

I'm as confident as can be that the bags are not the problem.  And, the tangs can't be forced apart because this rifle has the CPA through bolt.  The upper and lower tangs are connected together by by a substantial post that is attached by machine screws to both the upper and lower tangs. 

My best guess is that tensioning the mainspring causes a moment about the point where the through bolt post attaches to the lower tang.  It may be that the hole (the relief for the attaching screw) in the lower portion of the post is a bit oversize which allows this moment to move the bottom of the post a few thousandths.  This movement results in a very small, but significant displacement of the buttstock relative to the action.  Releasing tension on the mainspring relaxes the force on the post and the buttstock. 

JackHughs

In your initial post, you state your rear bag is packed firm. It's been my experience and I'm not hardly the Lone Ranger on this, unless your rear bag is packed rock-hard, including the ears, you will get rifle movement by cocking the hammer. Just wanted to throw that issue into the mix as something you might want to check in your bench setup.

RSW wrote on Aug 29^th, 2013 at 3:31pm:


OK.  I actually have something available to test that out.  My wife is an American Rimfire Association (ARA) shooter and among her impressive box o' stuff is a one piece rest which uses a shaped delrin block to support the buttstock.  The next time she has the one piece rest out to the range, I'll set the CPA in it and check to see if the POA still shifts when the hammer is cocked.  Heck, I might even sneak in a few shots.

Thanks,

JackHughs

Randy has the right approach, my rear bag is rock hard and movement had just about deminished completely. My front bag is soft so I still get some minor displacement from it. It is near impossible to not displace the sand while cocking the hammer on any rifle that has one, it is just the nature of the beast but it does not have an ill effect on accuracy. I would look else where  to find the underlying problems with your verticle issues.

Quote:


Hi Jerry,

Thanks.  It's exactly the response I've hoped for since it makes addressing this issue less of a priority.  However, it's still on the list of things to work on.  Having the buttstock flex while the hammer is moving toward the firing pin cannot be a good thing.

JackHughs

FWIW, I have been struggling with an original 44 1/2 in .22 Short on the bench and I have the same problem and have asked myself the same question.   

The rifle I am using does not belong to me.  It is a very high quality rifle and I am being ultra-careful not to harm it.  I built a wooden crib (box w/o top or front end) to hold the stock and have padded it with a bath towel. With the towel in place, the stock is pushed into the crib until it is held firmly in place. Seems to me this is a low tech, soft approach but similar to the delrin block. 

This week's results:  I can get two consecutive shots into the same bullet hole IF I do not touch the buttstock after alignment.  Alignment is made after hammer is cocked & trigger is set.