question about loose base to action fit

[taliv]

i have a rifle with a custom action. round top. 4 screw holes with the star heads on them. when my gunsmith built the rifle, i asked him to also pin the base to the action, which he did.


i recently started experiencing stringing which made my best guess (after eliminating a few other variables) that the base must be loose. after all this gun has seen a lot of use.

so i removed the scope and started to remove the screws, thinking some of them must be loose to allow the base to shift.

i couldn't budge the first screw.

i sheered the bit head off in the second screw.

after some profanity, i got another bit, which i sheered off in the 3rd screw.

i relatively gently verified the 4th screw wasn't loose. and the pin hasn't moved.


so my question (now that i have half the screw heads filled) is given all screws are tight, and the pin is in place, would it be possible for the base to still be shifting? given it is a round top, i'm thinking the answer is no.

 

[rcmodel]

Seems very unlikely, unless the screws were bottomed out in the holes, and not in the base itself.

When you put it back on, put lock-tight or Accra-glass on the bottom of the base to 'glass-bed' it to the round action and be 100% sure.

rc

 

[Jim Watson]

Sounds like the bottom radius of the base is greater than the top radius of the action, giving only a narrow line contact down the middle.
As rc says, you can glass bed the bases to the action and eliminate that line contact and wiggle.

There is no sense to setting up on screws that tight. A guy here does that and makes changing stuff out an awful bother.

Or they may be glued in which can be dealt with by heat.

 

[taliv]

Just to clarify this worked just fine for years and has recently had a shifting zero. I don't think it's the base. I'm just trying to rule it out.

 

[rcmodel]

Maybe the scope went south??

Right after bedding and loose screws, that would be the usual suspect.

rc

 

[taliv]

yeah, i took it apart and checked bedding, cleaned it thoroughly (which i find morally offensive) and reassembled. shot 10 rounds. still ugly.

tried fixing the base, no joy.

just swapped scopes. only fired 5 rounds and ran 200 yards between shots to the target to find my impact (since the scope wasn't zeroed and the target was full of holes. i couldn't see the pen marks i'd made in holes to tell which hole was new.. i had to run because the sun was setting and i was rapidly losing daylight) still got 3 in one hole and 2 others about .75"

could be luck though. i'll try another 5 rounds tomorrow, but i'm hopeful at this point, i've isolated the problem to a $3700 POS S&B PMII.