Advice on slotting underlugs for accuracy

[wetzel]

I'm pretty new to gunbuilding, but have completed a few.  When using underlugs and pins I have read from a few places that you should make a horizontal slot to allow for the barrel to move back and forth slightly.  This is supposed to help in accuracy.  Can anybody comment on this?  How big of groove do you make?  Should you do this on a swamped barrel? 

[California Kid]

Not for accuracy but to allow for expanding and contracting wood.

[omark]

make the slot just as wide as the pin, of course. then, 1/8 inch fore and aft of the pin should be plenty.  mark

[BJH]

The real reason for slotting underlugs is to allow for wood movement. A really curley piece of maple can elongate a surprising amount from dry winter conditions to humid summer.  The rear lug 1/16 on either end front and middle lugs a fat 3/32 or so. I have seen a gun that was built with the nose cap 1/16 or so from the muzzle in winter, have the fore end elongate so the nose cap was a bit proud of the muzzle of the gun. This was a really curly piece of medium hard maple.

[LRB]

  This is done for accuracy as well. If the stock moves with unslotted pin holes, it will bend the barrel enough to throw your shots off. You may still shoot a tight group, but the POI will change. I have experienced that, and slotting the pin holes cured the problem.

[Jim Kibler]

Another factor to consider besides wood movement with changes in humidity is expansion / contraction of the barrel itself.  A 100 degree F change in temperature results in a change in barrel length of approximately .032" for a 48" barrel.  Not a huge amount, but plenty in a situation where it's pinned solid.  Considering winter to a barrel out in the summer sun, a 100 degree F change is not unreasonable.  I first noticed this sort of thing when setting a stock out in the sun to dry with the barrel installed.  It was a smoking hot day with bright sun and the barrel got very hot, too hot to touch.  In this gun, the nosecap was fastened directly to the barrel with a screw.  When the barrel heated up a slight gap developed behind the nosecap where it meets the wood.  When I brought it inside and it quickly cooled down, it tightened back up.  This is what prompted me to do a little math to see how much a barrel migh change in length.  Pretty suprising to me.

[wetzel]

Thanks you guys, that is exactly what I was looking for!!  This may sound dumb, but what process do you use to slot them?  A little file?  A cut down hacksaw blade?  Just curious.

[Dphariss]

I have used staples for keys and pins. Thus no need to slot.

Dan

[Jim Kibler]

Thanks you guys, that is exactly what I was looking for!!  This may sound dumb, but what process do you use to slot them?  A little file?  A cut down hacksaw blade?  Just curious.

Drill a hole on each side of the pin hole, connect them with a jewelers saw and then use a file to true things up a touch.  It helps to use a slightly smaller drill on each sides of the main hole to account for slightly misplaced holes etc.  You can grind down a  needle file if necessary to help it fit in the rough sawed slot. 

[wetzel]

Okay, that makes a lot of sense, I am going to do that from now on.  Why don't they show that in the video's I've seen?