I bought a used Spencer Carbine from a reenactor last year. I've had fits getting good groups with it at 50 yards (not messed with any less yardage). From a clean barrel, the first three or 4 rounds are near point of aim. Then, it begins to string low and right and finally settles about 6" low and 6" right. Clean the barrel, and it repeats.
I've free-floated the barrel. The only place the forearm touches the barrel is at the point where it is screwed to the barrel lug. I can easily pass a dollar bill between the forearm and barrel, even when the barrel is very hot.
I'm convinced it is either a fouling issue, or the barrel changes harmonics when hot....note the barrel has a chance to cool when I run the cleaning patches through it.
I'm shooting a 350 gr FP 0.513 diam soft bullet from Buffalo Arms. It has a single, but large lube grove, and is lubed with SPG. This bullet is in Starline Brass over 32 grains (by weight) of 3 F Olde Eynsford. On a good and humid day (typical in Texas) and 21 shots, a patch runs through the barrel fairly easily, but I do get a bit of crud...nothing that is alarming to me.
I would like to be able to get 21 shots out of it and it hold a decent group! Any suggestions will greatly be appreciated.
Humbly,
Good Troy
AKA Two Dogs