I purchased and sold one of the first Marlin 375s. I highly recommend the 45-70. Here is why.
There are a number of problems related strictly to the 375 Winchester cartridge:
1. Accuracy or lack thereof. The best group I ever got from the gun was about 6" at 100 yards, no matter what. Several gunwriters have commented similarly.
From different days at the range with my 375 Marlin:
2.2”, five shots, 100 yards, 200g Sierra (6/23/2002)
2.0”, eighteen shots, 100 yards, 220g Hornady (2/17/02)
1.9”, five shots, 220g Hornady (9/12/2004)
2.0”, five shots, 220g Hornady (4/22/2006)
2.4”, four shots, 220g Hornady (7/30/2006)
Accuracy is a real problem...
2. Winchester brass is much too thin, with a severe tendency to dimple when re-sized. You must use very very little lube.
3. Brass tends to stretch more severely than any case I have ever seen. Brass has to be trimmed very frequently. Stretching also affect crimp, requiring resetting of crimp/seating dies on every batch. If not, the case crushes.
All the brass for my leverguns is kept to very close tolerances to avoid crimping issues. .30-30. .375W, .45-70, .44 Mag, same-same. I use Winchester ammo for the .30-30 and .375, no significant problems.
4. Hornady changed the design of their 220 grain bullet, causing a severe pressure change with the newer design.
Only if you overload it...
The SAAMI pressure standard for the .375 Winchester is 52,000 CUP.
H322, 2236fps, 48,900 CUP, Hornady 220g, Hodgdon Annual Manual, 2004, 24” barrel
H4198, 2233fps, 49,300CUP, Hornady 220g, Hodgdon Annual Manual, 2004, 24” barrel
AA2015, 2512fps, 41,500 CUP, Sierra 200g, Accurate #1, 24” barrel
AA1680, 2372fps, 44,800 CUP, Hornady 220g, Accurate #1, 24” barrel
Reloader #7, 2259fps, 42,100 CUP, Hornady 220g, Lyman #48, 20” barrel
5. There is one and only one powder for the 375 and that is RL7.
See above. Most folks I know think AA1680 is the better powder.
6. While the Marlin 375 is a great little rifle, it simply isn't worth all the hassle. It is accuracy limited to about 125 yards, despite relatively high velocity.
The three –shot, 2.9” group I shot at 200 yards was an anomaly – the wind was cooperating that day. The coyote I shot with it at 195 yards didn’t know what hit it, nor did the buck antelope I hit at 167 yards.
Took it down to the NRA Whittington Center one day for a bit of fun and ended up at the steel silhouette range shooting the 500-meter steel rams. Once I dialed in the drop and correction for the 40mph crosswind, the .375 Win knocked down 4 rams with my last 5 shots. FWIW, the .45-70 also did 4 of 5 with the last 5 shots. Not much difference that I could tell...
Guess I ought to stop shooting the .375 at 300 yards since accuracy is only good to 125 yards or so...
The 45-70 exhibits NONE of the problems I had with the 375 and is far more powerful with appropriate loads. Excellent accuracy. Little or no case stretching. No crushing or dimpling during reloading. No weird pressure spikes. The 45-70 is my all time favorite rifle cartridge.
There is no doubt the .45-70 is the more powerful cartridge. The problem I have with the .45-70 is that to get it to shoot as flat I have to deal with substantially more recoil. Much as I love my Marlin .45-70, the .375 Win does a great job.