I have a barrel for .50-140 Sharps for the Thompson-Center TCR-87 rifles. It was made from a turned down military .50 BMG barrel, and has a bore diameter of 0.507-inch. I quit shooting 0.510-inch Barnes 600-grain Orginals in it out of concern about the tight bore, and switched to smaller diameter paper patched lead bullets. How much would the 0.003 over-sized bullets raise pressure?

There is no way to tell without a strain gauge.

George

Work your loads up from start data and there shouldn't be any problem. For comparison, I have a fluted 270 Win H&R Ultra that is overbore, slugs .2795" groove diameter, .277" bullets shot 3" 100yds groups, the barrel also didn't fit the frame well, so I refitted it to another frame by milling the chamber face back(basically setback) and fitted it to another frame, that left it with negative headsace, so rather than rent a 270W reamer, I rented a 270W improved reamer and rechambered it to Improved. To deal with the overbore issue, I use .284" bullets, it shoots 130gr Speer handloads rather well, <1" 100yds groups @ 3225fps, no issue with pressure at all, I did have to turn the necks a little for the bigger bullet tho.

I also have a 50-140 H&R Buffalo Classic that I had JES rebore and rechamber from 45-70, I told Jesse I wanted to shoot .510 jacketed in it, slugs from the bore are kinda hard to measure due to the 5-groove configuration, but using a .004" steel shim wrapped around the slug I get .509", I've shot 570gr Hornady solids with smokeless Quickload 30kpsi based loads at just over 2200fps base with no issues other than tremendous recoil. I don't know for a fact, but I'm pretty sure the TCR is a much stronger action than the H&R, so you shouldn't have any problems with the TC.

I have 600gr cast to use in the 50-140 sooner or later, the only reason I worked with the Hornady jacketed is cuz they were on sale cheap for $30 a box, great for barrel break-in, although it didn't really need it, Jesse does excellent work.

wrongtarget - I loaded the 600-grain Barnes to about 1,600 fps with 115 gains of AA8700 and 575-grain paper patched lead bullets to about 1,625 fps with 110 grains of AA8700. The 575-grain paper patched ones do about 1,900 fps with 107.5 grains of AA3100, but with more recoil aren't that fun to shoot. I also shot some 720-grain paper patched bullets at 1,750 fps with 115 grains of AA8700 (also with a lot of recoil). Note that the heavier bullets shot faster than the 600-grain jacketed bullets with the same powder charge. Maybe the heavier bullets made the very slow burning AA8700 burn better? Between the .50-140 and shooting a 10-gauge barrel on that gun the stock split and I replaced it with a fiberglass stock I made. That stock shouldn't split like the wooden one but even after only 5 or 10 shots with the 1,625 fps loads the fun wears off. In addition, the lead bullets are made from cast cylinders finished out in a hammer swage and it takes about 40 really hard strikes with a heavy hammer to to get the bullets made. I would guess that between casting bullets, hammering them to shape, paper patching and the rest of the handloading effort it takes me at least an hour's effort to laod 5 cartridges. - DON

Lead boolits need to be from .001 to .002 (thousandths) larger than groove diameter.

Id be bit suspicious of a real .507 Groove diameter.50 BMG barrel. It's easy to misread a micrometer measurement of a sold lead swaged slug pounded down a bore. I make all my brass bullets to published groove diameters and never allow them to go more than 0.0005 over that dimension. Some bullet makers intentionally make all their bullets o.oo1 under groove diameter. For cast bullets the rule is o.oo1-0.002 over grove diameter and I do the same with paper patched bullets. How much pressures would rise are anyone's guess, but i doubt it would be catastrophic unless your at absolute max loading. I personally have shot a 6.5 jap rifle that was re-chambered in the 1960's for a .30-06. The bullets all swaged out and got pretty long, but that rifle had literally hundreds of rounds through it.The recoil was noticeable but I shot two deer with it. It's been the subject of a great deal of conversation at the local gunshops. -Rob