375 H&H Mosin

45flaco · March 16, 2020, 11:47 AM

Hey guys. Recently bought two mosin barreled actions, both with sewer pipe barrels. I happen to own 2 0.375" barrel blanks, and the equipment to load 375 H&H.
My plan is to put the blanks on the mosins, and chamber to 375 H&H for one, and 375 winchester for the other. The H&H is going to likely be a single shot, as I tried to fit one in the mag, and it didn't even come close to fitting, although a 300 Win mag will.
I plan to usually load around 375 flanged mag pressures, and only use the full power stuff for hunting.

I found that the blanks need to be threaded 1x16 TPI to fit the mosin receiver, by trying to thread random barrels into a spare receiver stub until one fit (ruger M77). This part should be relatively easy.

My other problem is bushing the bolt for the smaller rim diameter of the H&H. I've bushed a bolt before, but it was a much larger difference.
In this conversion, I will need to go from a 0.565" rim to a 0.535" one. That is a very small difference, but nowhere near small enough to ignore. The only time I have ever bushed a bolt was going from a 0.473" to a 0.378", a much bigger difference.

Ideas, suggestions?

Jim Watson · March 16, 2020, 12:43 PM

Can you get .375 Flanged brass? Rim diameters are close.

If it is going to be a single shot, it would look nice if you discarded the bottom metal/magazine and put on a plain trigger guard.

SHR970 · March 16, 2020, 04:37 PM

Instead of 375H&H have you considered your own version of the 375 Chatfield -Taylor? If a 300WM will fit chances are good that this will too.....and you can easily achive 375H&H power levels with it.

Jim Watson · March 16, 2020, 05:20 PM

9.5x54R would be a natural but reamer and dies would run up the cost.

45flaco · March 25, 2020, 09:35 PM

I had considered the 375 flanged, but like with the other cartridges mentioned, brass was the main drawback. I can get once-fired 375 H&H brass for right around a dollar a round locally, whereas the flanged brass is over 3 dollars a round at best. The other ones don't have available brass at all. I did originally consider 9.5x54r, but the complete lack of support for it stateside ruined that. One of the attractions of 375 H&H is that I know of 4 places locally that carry it. While I plan to reload, I do really like having guns that I can buy factory ammo for locally.
I will look into the Chatfield-taylor, but I suspect ammo or brass for it is going to be similar or worse than the 375 Flanged situation.

45flaco · March 25, 2020, 09:36 PM

I was already planning to ditch the bottom metal, and hoping to make a trigger guard that looks similar to the old SxS shotguns, that goes back with the stock a ways, and curves around at the front. Much better look than the military mag box.

Paul B. · March 26, 2020, 03:31 PM

"Instead of 375H&H have you considered your own version of the 375 Chatfield -Taylor? If a 300WM will fit chances are good that this will too.....and you can easily achieve 375H&H power levels with it."

I was going to suggest the same thing but you beat me to it. I like mine very much and it's quite accurate.

"I will look into the Chatfield-taylor, but I suspect ammo or brass for it is going to be similar or worse than the 375 Flanged situation."

Ammo is no problem. I've made brass for mine with everything from 7MM Rem. Mag. (last choice BTW) to .458 Win. Mag. (first choice.)Easiest is either use the base .338 Win. Mag. brass and just neck up or my preference, .458 Win Mag. brass Either will work with one pass through the sizing die with a very slight trim of the neck to insure the necks are square. IIRC, there is a place that makes brass with the proper head stamp but unless you're hunting out of country where head stamp and barrel stamp must match it's not something to worry about.
Paul B.

SHR970 · March 26, 2020, 08:17 PM

If you insist on factory ammo as an option go 375 Ruger and call it a day.

TRX · May 31, 2020, 07:23 PM

.565 to .535, you're looking at normal breechface fit for a lot of guns. As long as the extractor has enough stroke to engage the extractor groove in the case, you're good to go.

If you're feeling doubtful, note that many 9mm automatics share the same slide as .40s, and that SMLEs don't even *have* a breechface recess.

There's a maximum of .015" case misalignment with the H&H rim in the Mosin bolt; the feed path won't notice a thing.

45flaco · August 13, 2020, 08:43 PM

recently got the project into shootable condition. Not done by a long shot, but it shoots. Couldn't wait, put about 150 rounds through it before I even got the sights attached. Put the front sight on today at work, and tried it out. Will have some minor fine tuning to do, once I go to a real range with paper instead of beer cans.
Currently still trying to get the magazine to feed, it gets hung up about 50% of the time.
Also, with the stock ejector I'm limited to 3.35" OAL, so I'm going to make my own. It could also use some modification for it's role in feeding. Pictures coming when I figure out how to use my phone.

45flaco · August 13, 2020, 08:57 PM

Had to reduce size of image to upload, but it may have hurt quality. If anyone has an area of particular interest, I can upload those too.

45flaco · August 13, 2020, 09:05 PM

I ended up going with what you suggested, and just leaving the slop in the bolt face. Hasn't hurt extraction, or the brass. Seems you were right. I've reloaded the same brass about 6 times now, and they're not showing any funny extractor marks.
I appreciate the advice and examples.

45flaco · May 2, 2021, 10:38 AM

Uploaded some videos to youtube, as they wont fit in the size limit for posting on here. One video of it cycling and ejecting full length loaded ammo, one of it firing.

https://m.youtube.com/watch%3Fv%3DAM...NjqxqxzKsUsNzW

https://www.youtube.com/watch%3Fv%3D...8gfdMBDVe3-YK_

March 16, 2020, 11:47 AM	#1
45flaco Senior Member Join Date: November 19, 2017 Posts: 134	375 H&H Mosin Hey guys. Recently bought two mosin barreled actions, both with sewer pipe barrels. I happen to own 2 0.375" barrel blanks, and the equipment to load 375 H&H. My plan is to put the blanks on the mosins, and chamber to 375 H&H for one, and 375 winchester for the other. The H&H is going to likely be a single shot, as I tried to fit one in the mag, and it didn't even come close to fitting, although a 300 Win mag will. I plan to usually load around 375 flanged mag pressures, and only use the full power stuff for hunting. I found that the blanks need to be threaded 1x16 TPI to fit the mosin receiver, by trying to thread random barrels into a spare receiver stub until one fit (ruger M77). This part should be relatively easy. My other problem is bushing the bolt for the smaller rim diameter of the H&H. I've bushed a bolt before, but it was a much larger difference. In this conversion, I will need to go from a 0.565" rim to a 0.535" one. That is a very small difference, but nowhere near small enough to ignore. The only time I have ever bushed a bolt was going from a 0.473" to a 0.378", a much bigger difference. Ideas, suggestions?

March 16, 2020, 12:43 PM	#2
Jim Watson Senior Member Join Date: October 25, 2001 Location: Alabama Posts: 18,542	Can you get .375 Flanged brass? Rim diameters are close. If it is going to be a single shot, it would look nice if you discarded the bottom metal/magazine and put on a plain trigger guard. Last edited by Jim Watson; March 16, 2020 at 01:39 PM.

March 26, 2020, 03:31 PM	#7
Paul B. Senior Member Join Date: March 28, 1999 Location: Tucson, AZ Posts: 3,802	"Instead of 375H&H have you considered your own version of the 375 Chatfield -Taylor? If a 300WM will fit chances are good that this will too.....and you can easily achieve 375H&H power levels with it." I was going to suggest the same thing but you beat me to it. I like mine very much and it's quite accurate. "I will look into the Chatfield-taylor, but I suspect ammo or brass for it is going to be similar or worse than the 375 Flanged situation." Ammo is no problem. I've made brass for mine with everything from 7MM Rem. Mag. (last choice BTW) to .458 Win. Mag. (first choice.)Easiest is either use the base .338 Win. Mag. brass and just neck up or my preference, .458 Win Mag. brass Either will work with one pass through the sizing die with a very slight trim of the neck to insure the necks are square. IIRC, there is a place that makes brass with the proper head stamp but unless you're hunting out of country where head stamp and barrel stamp must match it's not something to worry about. Paul B. __________________ COMPROMISE IS NOT AN OPTION!

March 16, 2020, 04:37 PM	#3
SHR970 Senior Member Join Date: January 24, 2011 Posts: 1,427	Instead of 375H&H have you considered your own version of the 375 Chatfield -Taylor? If a 300WM will fit chances are good that this will too.....and you can easily achive 375H&H power levels with it.

March 16, 2020, 05:20 PM	#4
Jim Watson Senior Member Join Date: October 25, 2001 Location: Alabama Posts: 18,542	9.5x54R would be a natural but reamer and dies would run up the cost.

March 25, 2020, 09:35 PM	#5
45flaco Senior Member Join Date: November 19, 2017 Posts: 134	I had considered the 375 flanged, but like with the other cartridges mentioned, brass was the main drawback. I can get once-fired 375 H&H brass for right around a dollar a round locally, whereas the flanged brass is over 3 dollars a round at best. The other ones don't have available brass at all. I did originally consider 9.5x54r, but the complete lack of support for it stateside ruined that. One of the attractions of 375 H&H is that I know of 4 places locally that carry it. While I plan to reload, I do really like having guns that I can buy factory ammo for locally. I will look into the Chatfield-taylor, but I suspect ammo or brass for it is going to be similar or worse than the 375 Flanged situation.

March 25, 2020, 09:36 PM	#6
45flaco Senior Member Join Date: November 19, 2017 Posts: 134	I was already planning to ditch the bottom metal, and hoping to make a trigger guard that looks similar to the old SxS shotguns, that goes back with the stock a ways, and curves around at the front. Much better look than the military mag box.

March 26, 2020, 08:17 PM	#8
SHR970 Senior Member Join Date: January 24, 2011 Posts: 1,427	If you insist on factory ammo as an option go 375 Ruger and call it a day.

May 31, 2020, 07:23 PM	#9
TRX Senior Member Join Date: September 24, 2008 Location: central Arkansas Posts: 400	.565 to .535, you're looking at normal breechface fit for a lot of guns. As long as the extractor has enough stroke to engage the extractor groove in the case, you're good to go. If you're feeling doubtful, note that many 9mm automatics share the same slide as .40s, and that SMLEs don't even have a breechface recess. There's a maximum of .015" case misalignment with the H&H rim in the Mosin bolt; the feed path won't notice a thing.

August 13, 2020, 08:43 PM	#10
45flaco Senior Member Join Date: November 19, 2017 Posts: 134	recently got the project into shootable condition. Not done by a long shot, but it shoots. Couldn't wait, put about 150 rounds through it before I even got the sights attached. Put the front sight on today at work, and tried it out. Will have some minor fine tuning to do, once I go to a real range with paper instead of beer cans. Currently still trying to get the magazine to feed, it gets hung up about 50% of the time. Also, with the stock ejector I'm limited to 3.35" OAL, so I'm going to make my own. It could also use some modification for it's role in feeding. Pictures coming when I figure out how to use my phone.

August 13, 2020, 09:05 PM	#12
45flaco Senior Member Join Date: November 19, 2017 Posts: 134	I ended up going with what you suggested, and just leaving the slop in the bolt face. Hasn't hurt extraction, or the brass. Seems you were right. I've reloaded the same brass about 6 times now, and they're not showing any funny extractor marks. I appreciate the advice and examples.

May 2, 2021, 10:38 AM	#13
45flaco Senior Member Join Date: November 19, 2017 Posts: 134	Uploaded some videos to youtube, as they wont fit in the size limit for posting on here. One video of it cycling and ejecting full length loaded ammo, one of it firing. https://m.youtube.com/watch%3Fv%3DAM...NjqxqxzKsUsNzW https://www.youtube.com/watch%3Fv%3D...8gfdMBDVe3-YK_