Load for 375 Winchester in Marlin micro-groove rifle

Howdy V-Kid:

Re: CB choices - any of those you have may possibly work.
The starting point I'd recommend though is to slug your barrel to see what you have to work with for groove dimensions. I've corresponded with a couple guy's on the CB-L forum that have the Marlins w/MicroGroove bbls. and they found that they needed to go to .381" to .382" diameter to get good accuracy. The real officianado's for CB shooting prescribe that the bullet should be as large in diameter as the chamber will permit (i.e. small enough to chamber the loaded round - but as large as possible). Personally, I've had pretty good luck with bullets sized about .002" over groove diameter - this is using CB at BHN 12 range.

Too small bullets can give rise to poor accuracy AND leading problems.

AA 1680 is a good powder choice because it allows you to get enough powder into the case (which has relatively small case volume) for higher velocities and develops does so at lower pressures than other powders. Alliant RL-7 is recommended by most of the loading books as being an excellent choice too although in my experience - loads using this powder seem to kick a WHOLE lot more than with other powders developing similar velocities.

Best regards-

Sky C.

I've fired LOTS of 250g LFNGC's from my Marlin .375's, both of whjch sport MicroGroove rifling. Best performance from both rifles was obtained with .377" diameter bullets, BHN 21, and contrary to popular thought regarding cast in MicroGroove barrels, at velocities over 1650 fps.

Many of our customers report excellent accuracy out of their Marlin .375 Winchesters using the above mentioned bullet, and a starting load of 32.0g AA 1680 with a Winchester LR primer.

Other powders that have given excellent performance with this bullet at near factory velocities are RL-7, H322 and H335.

Oddly, my guns haven't liked the Lyman #375244 bullet at all! I've tried different bullet hardness, sizing diameters and a variety of powders/velocity combinations with no gratifying results.... mediocre being the best performance I've obtained.

Too, I've found it essential to seat the bullet out of the case as far as the action will cycle the round, then crimp with a Lee Factory Crimp Die, regardless of where the case mouth falls in relation to the crimp groove. The die will tuck the case mouth into the bullet sufficiently and not allow it to set back in the case under recoil. Marlin, when they chambered these rifles used a reamer that cut ENORMOUSLY LONG throats of relatively generous diameters, and aloy bullets as a general rule don't like a long jump into the lands... the .375's are no exception to this rule as far as my experience bears out.

Lastly, my particular guns have an annoying tendency to throw their projectiles sideways at the target if velocities are under about 1500 fps. This I really can't explain, as the twist is tighter than nearly all standard twist 38-55's, and should stabilize the bullets with surety.... however, the targets remain to bear witness that sometimes logic doesn't always prevail!

I've had many reports back from customers using my above recommended loads and acheiving very satisfactory, to excellent accuracy. Especially in those barrels that have been fire-lapped to releive the constrictions where the dovetails are cut for the rear sight and forend hanger. Fire lapping the MicroGroove barrels (especially those that look like they were rifled by a blind Girl Scout with a cold chisel), enhances the ease by which superb cast bullet load development is accomplished.

Just my 2 Cents worth!

God Bless,

Marshall Stanton

Back years ago, I was a newly wed, had a brand new son, I watched my company go bankrupt, and on top of this I was hankering for a new shooter to play with. I went back to college for some more training, sold my 03a1 Springfield for some fast cash, and I bought a used .375 Marlin. This was in 1992, 11 years ago at a gun show and the new Marlin .375 came with a partial box of cartridges, 8 total loaded, and some emptys. I was able to buy for $15 a used set of RCBS dies, and I bought a Lyman mold, not the out-of-print 375296 that I wanted, but the smaller diameter mold intended for the .375 H&H. I forget the number, but it was a standard offering. I had a hard time finding anything other than factory Winchester loaded ammo, and even that was in short supply and very expensive at over $20 a box. A friend of mine gave me some 30-30 cases blown out to fit his custom chamered and rerifled 788 Remington. I still had to acquire the proper lubrisizer die and top punch and gas checks. These I ordered from Midway Arms. First time out, I loaded the cases with 2400 load data from the Lyman cast bullet book,and shot the bullets as cast, without the gas checks, and lubed with NRA Alox lube cookie cutter style. The results were dismal, every shot keyholed, but they went into a 6" group on paper at 100 yards. I had no idea of the velocity. The friend who gave me the fire formed cases said to use "real power", so I used some Reloader 7 that always was my favorite for the 30-30. By this time I had slugged the barrel, found it measured .377, which I felt then was somewhat oversized, as I expected it to measure .375 or .376 maximum. I ordered a .378 sizing die, and lapped out the mold so it would drop a .379 bullet and I headed for the range. It actually worked, I used a load that was right out of the Lyman book. The brass situation was something I had to work on, so I bought some .38-55 brass, a hundred cases and I planned to cut them down to .375wcf length and load with 38-55 data...until my friend Jimmie came on the scene..."Why not just check the chamber and see how long it is?" He had the idea (because the bore diameter was more 38-55 than his custom 788 in .375) that the chamber might be a 38-55 instead. I got out the materials to do a chamber cast and found out that it was, in fact, a 38-55 chamber. I got on the phone, called Marlin up, and talked with one of their engineers. I told him that the chamber length was a 38-55 and asked in fact, if they cut these 375 Marlins as 38-55's to keep potential problems down to nothing if someone shoved in a 38-55 cartridge. The engineer replied, "we haven't chambered a 38-55 in 63 years"..."This is the work of a kitchen-table gunsmith"...My next question was, "can you explain why the barrel is .002" oversize then?" The phone went dead... disconnected.......One more thing...Because of the tumbling, I fire lapped the Micro grooved rifle as Veral Smith of LBT recommended. I didn't think it could hurt. He said that cast bullets shot well in Marlins if they were fire lapped.

Since then I've read on this forum that Marlins are cut long in the chamber. My bullet mold that has been lapped out now gets minimally sized, I use Veral Smith's blue lube, and I use the maximum charge of Reloader 7 that is right out of the Lyman cast bullet handbook. The bullet, gas checked, and lubed weighs 270.3 grains, cast of my wheelweight and 50/50 barsolder approximation of Lyman #2 alloy. It goes into a 1.5" hole at 100 yards. I shot a 8 point buck with it on my family's farm on Beaver Island Michigan early on opening day in 1998 at 111 yards. I never recovered the bullet, the animal dropped in it's tracks.

I never used a different load in my Marlin .375 than the ones out of the Lyman cast bullet handbook using Reloader 7. I felt I had a great shooter so why try all of the possibilities?... I still have the 8 factory loads in the box, and since then have found some new win 375 brass. I never fired a factory cartridge, or anything other than a cast lead bullet handload in it.

This is an excellent cast bullet rifle, and I feel it will take all the deer I will ever want to shoot. The only problem is the lack of factory ammunition in this caliber and it's expense. But a handloader can make his own even if it means reforming 30-30 brass and using a cast bullet. History again repeats itself, the old 38-55 is now the 375 wcf.