The 38/55 is a fun shooter but bore sizes are all over the map.

I stopped at a yard sale and on one of the tables was a box of 38-55 LBT style heat treated solids cast bullets with a gas check. A count of 100 bullets 260gr sized .380 for $35. They were made by Cast Performance Bullet Company. I hope these will be the magic load for this old Winchester. I have the first 20 in the loading block and might start with Trail Boss 6.5gr. Will a gas check make a difference?

Quite often, a gas check design will be more accurate with a gas check affixed, but that depends on many factors. If you have the checks and a sizer that will crimp the check on the bullet base, try some loads with and without gas checks. Shooting some groups from a benchrest at 50 or 100 yards will tell you what you want to know. May shoot fine without the checks.