Swamped barrel accuracy question

[curly]

I have built a rifle using a GM "C" weight swamped barrel. The problem I'm having with this rifle is after 5 to 7 shots, the accuracy goes completely south on me. My thought is the heat dissipation is unequal causing the center of the barrel to cool off faster than the breech and the forend thus changing my POI. If thats the case, I've just built a $700.00 gun I can't use in competition. Anyone else have any problems like this or ideas on the subject?

Curly

[D. Taylor Sapergia]

I do not think your problem is in the barrel.  I have many longrifles with swamped barrels and none of them suffer this ailment.  I suspect it's your load or loading.  Pls. describe your load etc.

[Acer Saccharum]

Find your fired patches. If there are burn holes or tears, it may help determine the issue.

[retired fella]

Could also be your underpinning.  If too snug that could cause accuracy problems.  I'm with Acer I'll also bet that you are getting some blowby causing a bit of fouling.  Try wiping between shots to see if that helps.  Then you could start fooling with patch thickness and lubes.

[sqrldog]

One other problem could be stress on one of the center lugs if the barrel is slightly bowed by being clamped when you drill your pin hole. After several shots the barrel heats and stresses against the pin. Try removing the middle pin and see if it helps. Tim

[Ky-Flinter]

Curly,

I don't think uneven heat dissipation, by itself, would be causing the problem, but as mentioned earlier it could cause stress on the barrel pins/keys.  Are your underlugs slotted to allow for expansion/contraction?

My favorite rifle has a Rice B weight .50 caliber barrel.  It's thin-walled and light.  In 20 shot matches the barrel gets hot to the touch, but it doesn't affect POI.

-Ron

[curly]

Thanks for all the replies. Barrel is 50 cal, my load is 55gr , using a .0020 patch. I use a 50/50 solution of water and Balistol for lube. Patch is sound after firing, no holes or cuts. Nice tight fit, with about, roughly 600 shot thru it. It is the first time I've used wedges instead of pins and there is more than enough room for expansion from front to back movement. I will try and take that center wedge out and see if that helps me. Sounds logical to me. I do admit that it isn't fully sighted in for all seasons yet, but this is the second time this has happened to me on a woods walk. First 5, 6 or7 shots, which are rather difficult shots, are no problem. Everything is down hill after that. Thanks all for the suggestions, I'll work on all of them.
Curly

[jerrywh]

 Shoot that gun off the bench and see if it does the same thing. Maybe your just getting tired.  Don't be offended. A barrel doesn't expand much from a temp. change of 150°. Guns don't get that hot on a trail walk. I have a real nice flintlock that I just can't hole up very well any more. 

[bob in the woods]

What size ball ?  These barrels have .016 deep rifling if I recall correctly. You might need a .022 patch

[jerrywh]

 if you have a 42" long barrel and the temp increases by 80° F it will expand .053"
 I doubt if that barrel will increase 80° temp on a trail walk.

[curly]

The barrel is 36", sterling front sights,  and the size  ball is .495. So its a nice tight fitting ball. And your absolutely right. I will get out their and bench shoot until I find out whats going on with it. Thanks for all your replies, nice to know I have a place to go for help when needed.
Curly

[smylee grouch]

I think the GM barrels have square cut grooves around 12/1000 deep and I have never had a problem getting them to shoot when I used a tight ball/patch combo with enough powder.

[Daryl]

Thanks for all the replies. Barrel is 50 cal, my load is 55gr , using a .0020 patch. I use a 50/50 solution of water and Balistol for lube. Patch is sound after firing, no holes or cuts. Nice tight fit, with about, roughly 600 shot thru it. It is the first time I've used wedges instead of pins and there is more than enough room for expansion from front to back movement. I will try and take that center wedge out and see if that helps me. Sounds logical to me. I do admit that it isn't fully sighted in for all seasons yet, but this is the second time this has happened to me on a woods walk. First 5, 6 or7 shots, which are rather difficult shots, are no problem. Everything is down hill after that. Thanks all for the suggestions, I'll work on all of them.
Curly

Curly, that would be a .020" patch, not .0020" = typo.  You patch "sounds" good if it is a heavily compressed measurment, and Taylor shoots just such a combo himself in his .50 Virginia with rounded groove bottom Rice barrel.  If he shoots more than 85gr. 2F, (about 1,500fps) owever, he gets burns due to some blowby and must use a tighter patch for the hotter loads.   Minor blowby, evenw ith only 80gr. 2f, can easily be seen if it is happening. What is evident on recovered patches, are brown scorch marks (in the groove marks) on the recovered patch.  If that is happening, you may be building fouling in the groove bottoms, however with only 55gr. 3F, you should not be getting enough buildup of fouling that effects accuracy.  55gr. is way too light in my honest opinion for any sort of accuracy past 25 yards. About any powder charge should stack the balls at only 25 yards. It takes more powder to get accuracy at longer ranges.