I am having issues with loading for this round. I bought a set of Lee dies, I usually buy RCBS, but they were out. So I sized virgin Starline short brass and loaded 37gr of 3031 with a 240gr cast bullet. Bullets are claimed at .375, but actually come out at .380 diameter. HSM bullets by the way. Well I get a bulge on one side of the brass from the bullet which I find odd and they will not chamber without much effort. I found some old SP Hornady 225gr bullets sized a true .375. Of course they fit fine and the dummy round chambered ok. So obviously it's with the bullets? I found a load for the .38-55 in the Hornady manual for a 220gr jacketed SP. Why can't I reduce the load by a grain or two and shoot those?
Also should mention I just picked up a new production Cimarron Deluxe. I would assume the Hi Wall action would be fine for all but the hottest loads? 
TIA

I am not to that point yet, I just need to get some rounds down range as for now I am limited to store bought bullets.

Regardless of whether you just need to get some rounds down range, you need to get the right size bullets for your rifle.

Just like you wouldn't put 7mm bullets into a 270 rifle, (even though they're only .007" different) you can't put .380 bullets through a .375 rifle and expect it to work correctly.   

Since the manufacturer doesn't put the bore size on the barrel, just marks it .38-55; you need to check, or verify with the manufacturer if it's a new rifle.  It's as simple as carefully using a caliper in the muzzle to measure between the bottom of opposing grooves, or pushing a soft lead slug through the barrel, and measuring it.   

Then, buy the correct sized bullets for the barrel you have.  If the brass is too tight to accept those bullets, you will have to get a custom expander button for your die set, to expand it a bit more to fit correctly.

Are you crimping the bullet?  A bulged case can happen when seating a bullet if the crimp is engaged before the bullet is fully seated. You really don't need to with a single shot rifle if the bullet gets enough case tension to hold it in place.

OTOH a friend has an old Winchester model 94 chambered for 38-55 that had a bore diameter well over .380". It would not shoot well with anything available. When he tried to chamber a case loaded with lead bullets as cast, about .381", it wouldn't chamber. Had the chamber reamed out to accept the larger bullets but he rifle still didn't shoot worth a hoot because the bullets ere too small. I think he now is having the barrel relined. That is not an uncommon issue with old Winchesters.

I can't imagine a new Uberti having that problem. Hope you get it figured out.

John

Absolutely agree with GB and add the bullet will not withstand the velocities the OP is to get with such powder charges. Keep the speed below 1300fps and you should see success without leading the bore and having good accuracy. A reasonable load would be 9-10 grains of Unique powder. Good luck.

This should help:

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good luck.

Piltdownman wrote on Aug 23^rd, 2022 at 3:02am:


Apples to fish comparison.  This guy is trying to get factory purchased bullets loaded into his brass using factory dies to use in an unknown bore/chamber rifle because he "needs to get some bullets downrange".  He isn't using a custom rifle with a special designed tapered leade and chamber with specially swaged tapered bullets to match.  He isn't buying a special reamer to change his rifle to match his bullet.

Within the constraints of an unknown rifle bore/chamber and commercial bullets and brass and loading dies, to make the rifle work you have to determine what bore/chamber you have and buy bullets to fit that, and buy an expander that will make your dies fit that bullet.

Or, jump up a couple of sophistication levels, and design and implement a custom cartridge/bullet/chamber reamer/barrel combination as you've described above, fit your brass and custom dies to that combination, and experiment until you make it work. Nothing in the OP's post indicates he has the desire or knowledge to do that.

Not offended, and I may or may not have experience doing something similar; that's not applicable to the OP's question.  What I try to do is give information that will work for the person answering the question; that are within the parameters that seem to apply from their question.  We, who have a lot more capability and experience than many that come here asking questions, often give answers that can cause those less experience members throw up their hands and walk away instead of joining in and being the next generation that carries on our single shot rifle traditions.

Not all of us have the experience, time, money and knowledge to design custom cartridges, reamers, and bullet molds to meet particular objectives; nor does everybody want to.  I try to give applicable answers to those that are asking the questions.   

I can make a hell of a lot of things work, including improperly sized bullets.  But, I recommend to someone trying to get a simple cartridge and rifle to work with the objective of "just get some bullets downrange" to determine the correct size for their rifle, as is, and acquire them rather than develop a special chamber and bullet mold and system and get custom components made to implement the special system.