Initial Reloading Notes for .375 Cheytac for DTA HTI

[secondofangle]

I have found little specific information for loading this even after cruising the forums for 2 months. I'll post what I've decided to do and execute some loads today with all the background information both as a record for me, and for others who may be working up their own loads.

Background: I've got about 140 rounds through my DTA HTI so far, about 70 rounds of DTA munitions 352 grain, about 50 CheyTac 350 SMKs and about 20 Cheytac 350 grain "balanced flight projectiles". Without getting into gory details, the 350 SMK from Cheytac shoot the best (very close to 1/2 MOA) and I'm interested in a long range hunting load, and even though these don't perform very well on deer past 1000 yards, I decided to go with the SMK because of their accuracy, their field confirmed velocity of 3150 FPS, and the fact that I got 500 of them from CTD for $350. I await an AMAX or an Accubond offering that will have a similar or better BC.

I tore apart a factory CT load with 350 SMK and found that whatever powder is inside weighs 136.5 grains. I think it's Retumbo. The OAL of the cartridge is 4.100". I consulted Quickload with all my cartridge and gun specifics, and if this is Retumbo, its not at max pressure, although this depends critically on your "Shot Start Pressure" which I left at default because these are seated deep and are not on the lands. You can barely hear the powder when you shake these loads so it's just under compressed.

BRASS: I got 100 Jamison from CT for about $2 each. I hand chamfered the inside and outside of the case mouths because my motorized RCBS thingie sucked at the job on these large brass. This brass does not appear to be annealed. I'm not sure if this is worth it because I have been told by a trusted guy at DTM that the primer pockets will be the rate limiting factor for my brass life. The DTM Bertram brass does appear to be annealed, however. I do not have a reloading tray that will accommodate the case head of the 505 Gibbs based 375CT round, so I used the box the brass came in. I cannot find a tray yet that is stated to accommodate this case head.

RESIZING: I used a 50 BMG RCBS press with the adapter (product number 88715) for the #47 shellholder, and I'm using Dave Viers Viersco dies with a diameter of 1.5". They're quite nice. I found that the expander ball was difficult to run through the neck on the first couple of brass. Subsequently, in addition to generously lubing the case body with RCBS case lube, I lubed the inside of each case neck individually with RCBS case lube applying it with a Q-tip. This made the first full length resizing much easier. The case length is 3.030 +/- 0.005. I tested an empty case and it chambers easily. I have the press mounted to a hand made bench that I made using 2 layers of 3/4" plywood 4'x4' glued and screwed togehter as the top with a 2x6 skirt that is bolted to 4 4x6s for legs with a bottom shelf (only one layer of 3/4" plywood on the bottom shelf which also has a 2x6 skirt for bolting to the legs) fastened together with 7" 3/8 bolts, 2 per level per leg. It is SUPER solid and loaded with equipment weighs over 400#. It is UBER-OVERENGINEERED-SOLID and I'm glad it is because this press for this cartridge puts a lot of stress on the bench. I noted minimal flex. I built this for 50 BMG reloading that I have never got around to doing yet. But the press is perfectly adapted for 375 reloading.

PRIMING: I used an RCBS bench mounted automated priming thingie. I have been told to worry about primer pocket expansion with multiple reloadings. I deprimed a factory CT shot case and reprimed it. The primer seated with mild-moderate force. I'm guessing that after just one firing the primer pocket is indeed expanded a bit, especially after comparing to priming the new Jamison brass. The Federal 215M primers seat "stiffly" in the new brass, but not as stiff as new Nosler brass in my 300 RUM. Those pockets seem tighter, from the factory.

POWDER CHARGING: The RCBS powder hopper maxes out at 120 grains of Retumbo. Fortunately I have 3 of them. So I maxed out one and set the other at about 10-12 grains and threw 2 charges, one at 120 grains, the next at 10-12 grains and then hand trickled the rest of the charge onto an RCBS ammomaster electronic scale (which is excellent and WAY better than the Lyman offering I previously used, BTW). I then used a Q-tip to push the few grains of powder that were stuck to the residual lube in the case necks down into the rest of the charge in the case. Note in the photo the powder dispensers: the one in the center has the plunger fully unscrewed and the one on the right nearly fully inserted.

I reloaded 12 shells to start, 3 each of 134.0, 135.0, 136.0 and 136.5 grains.

BULLET SEATING: I used a factory CT round to crudely adjust the the seating depth to 4.100". I then seated the bullets. The Viersco micrometer (an option that I selected on his dies) works SPOT ON for an individual round, meaning that if you seat and it's 4.105, and you adjust the micrometer 0.005, it brings it right down to 4.100. This is better than my Redding dies do. In spite of this, the OAL of the loaded cartridges varies by 0.015 to 0.020, probably because of the meplat of the SMKs which are irregular and which I did not uniform. I then noticed that I don't have a .375 anvil for my Hornady OAL gauge, so I cannot measure base to ogive for uniformity. I ordered the anvil and this will have to wait for a subsequent reloading session. I'm going to wager that the base to ogive is far more uniform than 0.020

I used a Hornady concentricity gauge to check run-out. They are all within 0.0025". Usually I will strive for 0.001, but I found that this piece of equipment does not "true" the round as well as it does on smaller calibers, so I said "it's good enough" and let it rest. The Viersco dies have a sliding sleeve that aligns the bullet and guides it up the die like Redding Competition dies, so maybe that's why they were relatively concentric right out of the seating die.

If I get a chance before SHOT, I will test these rounds at the range and report back, hopefully with photos.
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[CthulhuAwakens]

tag for later

 

[cowboywhorses]

Nice job

 

[AZ.noob]

The Cheytacs and 50 BMG are the only cartridges my local range prohibits (along with all tracers, incendiaries, etc). Probably 416 Barrett also, come to think of it.
Supposedly the berm may not be capable of containing them. WOW

 

[secondofangle]

My to most accessible local ranges also ban 50 BMG for a litany of poorly considered reasons....but the Cheytacs are OK.

You should show up with a 20mm Vulcan and tell them "it's not on the list"

 

[secondofangle]

OK, I went to the range today.

The testing showed that I have an apparent "node" at about 136.0 grains. Granted, that was shooting only 3 shot groups at 134, 135, 136, and 136.5 grains. But I felt that , given the cost of this experiment, that I would settle on 136 grains. It shot about 1 MOA at this load. BUT, it was cold as hell, I was shivering, the wind was 10 MPH and constantly changing and I was not totally satisfied with the bipod/monopod and my rear rest on an icy bench, etc, and I allow that I was not shooting at my best all day. And that my best is not close to "the best".

Tonight, I loaded 20 more rounds at 136.0 grains. But, compared to my test loads with an approximate OAL (see above) of 4.100, I made the following batches of 5 rounds, varying the seating depths using the micrometer on the dies:

5X with OAL backed off the micrometer by 0.020 for an OAL of ABOUT 4.120
5X backed off 0.030
5X backed off 0.040
5X backed off 0.050 with an OAL of give or take 4.150. These still do not appear to touch the lands, based on chambering a round and finding no rifling marks and no change in OAL after unchambering and remeasuring the OAL.

Consulting Quickload, increasing the OAL as an isolated variable decreases the pressure because the case volume filled is less. But, the initiation pressure may increase as I approach the lands which may counteract the former effect, but in unknown and unmeasurable ways.

I'm ASSUMING that I will still be under pressure as the OAL increases. I will shoot the batches in 5 shot groups with increasing OAL and look for pressure signs as I also look for changing accuracy and measure velocities with decreases in seating depth with the 350 SMK.

I will report back. If I get some results that I'm not ashamed of, I will post pics.

Load development is a bitch.

 

[secondofangle]

Here's the target from the above loads. Windy again, but I felt like I was shooting pretty good. I'm somewhat disappointed in the accuracy results. The best group on the bottom right is 1" center to center. I was hoping for .5" - but I'm used to shooting 3 shot groups.

What's a good 5 shot group diameter?

I don't know where to go from here. I did do some experimenting when I got home and loaded a dummy round to determine where the lands are. They appear to be touching the lands at an OAL of 4.210 which is a full 60 thousandths from my longest loads from today's test.

When I get home from SHOT, I think I might load some at 4.200, 10/10000ths from the lands and see what happens in that area.

I'm AMAZED that I can find no information on the web from others developing a specific load for this. Starting from scratch suuuuuucks....

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[AZ.noob]

My to most accessible local ranges also ban 50 BMG for a litany of poorly considered reasons....but the Cheytacs are OK.

You should show up with a 20mm Vulcan and tell them "it's not on the list"

If I could, I most certainly would LOL
And, congrats on your testing results

 

[secondofangle]

OK. I will say that I am not yet satisfied. I want this to be a consistent 0.5 MOA gun and load....but I think I may be on the right track. This week, I loaded some loads that are about 10 thousandths from the lands (OAL about 4.210 in my gun, but with +/- .020 variability because of teh SMK meplats) with 132 adn 133 grains of Retumbo based on my experiences and investigations of the Cheytac USA factory loads that have about 136.5 +/- 0.7 grains of powder in them and which I had 4 of them blow up and perforate the case, that's another story. Here are the results with the SMK reloads at 132 adn 133 grains. The 3-shot groups average about 1 MOA. I'm not satisfied.
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Today I took out some loads that I had loaded to an OAL of about 4.220 which is right on the lands, at 132 and 132.5 grains (in the green target in the photo, I mistakenly labeled it at 133 but it was really 132.5 grain Retumbo loads). The best 132 group is just under 1" which Center to center is about 0.62 MOA. I'm still not satisfied. I shot the final 3 shot group with 133 grains retumbo at 300 yards and it was just under 3" - so about .75 MOA.

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At 300 yards, the 133 grains shot dead center elevation with 2.75 MOA adjustment up which implies 3150 FPS which I think is too high.

In the next few days, I will take some 130 and 131 and maybe some 0.5 grain in between loads out and test them. I really want a 0.5 MOA load for this gun. After I find it I will chrono it. If I'm above 3000 FPS with this bullet, I'll be satisfied.

 

[BoltActionSean]

Great work!

 

[secondofangle]

Now we're making some progress.

I got my 375 insert for my Hornady OAL gauge kit which measures base to ogive. Previously I was using calipers to measure COAL.

I also got a tap (5/16-36 IIRC) to tap the base of a case and use the hornady OAL to measure the distance to lands. Turns out that using this device, it is 0.010" further than I thought which corresponds to a COAL of 4.230 NOT 4.220 as I previously thought.

When I load the bullets to this depth, the bolt closes a bit hard and there are rifling marks on the bullet, BUT, the base to ogive remains the same, meaning that closing the bolt is not seating the bullet further.

The base to ogive distance is 2.391".

I loaded up 6 each of 131 and 132 grains Retumbo. And I learnt this:

That before, I was getting different COALS which I thought was due to imperfections in the meplats of the SMKs. So I was seating them less deep and using the micormeter on the Viersco dies to individually seat the bullets to the same COAL +/- 0.001. If I did not they would vary by COAL by up to 0.020.

Today, once I got the seating depth right on the first one, I found only 0.0005" variance in the base to ogive length (2.391" +/- 0.0005 - yeah, 5/10,000ths!). So I'm guessing that the Viersco dies index off the ogive, as they should. BUT, my Redding Competition dies do NOT give consistent base-ogive lengths with an AMAX for some reason, so DON'T take this principle for granted without confirming.

I then measured concentricity of each round to 0.002 as usually is my custom.

Now, here's the other thing - I read here somewhere recently that you should always shoot the 375 from the prone. I had been testing from a bench. So today I fired prone, the first 2 groups with the monopod (and the first one on a foam mat rather than directly on concrete for the last 3) and the second 2 with a small rear sandbag. I also tried my damnest to shoot as well as I could today.

I have been told not to mix jacketed bullets and copper solids, which for a while I had been doing at the range since I have a bunch of factory ammo I've been screwing around with along with my reloads. So before today's session, I cleaned the shit out of that barrel. I brushed it like a mofo, used 3 kinds of copper removers, and some JB paste and some Shimichrome. It was clean as a whistle for today's session. Which is why I was interested in the first cold bore shot.

Finally, after that SUPERCLEANING, I got out the torque wrench and checked the barrel bolts on the HTI. I found that the outer two bolts had loosened. I retorqued them to 80 inch pounds. Whether this affected previous results is anyone's guess.

Here are the results. Center shot is one I loaded too deep, used it as a cold bore shot. Groups on the right are 131 grains, on the left are 132 grains. No pressure signs, just a faint ejector mark, which is almost identical to what I got loading 136 grains 70-120 thousandths off the lands. According to Quickload, if I compare the deep seated rounds with 136 grains and an initiation pressure of 3000 and these touching the lands rounds with 132 grains and an initiation pressure of 10000, the pressure and the velocity should be almost identical. BUT, it's predicting 2925 FPS. When I chrono'd the 136 grain loads, they were cooking out at 3100-3125fps (as were the factory CheyTac rounds that were blowing up on me).

I sure hope these chrono about 3000-3100, because if they do, I'm going to stop and call it a done deal.

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[Dogtown]

Looks like you're in the ballpark. Great way to document your process, BTW.

 

[jbrand11]

tag

 

[secondofangle]

For those who don't want to read the whole thread, I will summarize some things that I have learnt that go beyond the usual "what stuff do I use to reload this cartridge in this gun":

1.) Shoot from the prone on a solid foundation with a good bipod and a good rear rest, whatever that means to you.
2.) Assure that your barrel screws are properly torqued and all the other checks of your equipment that are often taken for granted.
3.) I "believe" that the SMK wants to be close to the lands. Factory ammo is not likely to be close to the lands. You cannot know unless you measure your own chamber, and doing so may save you hundreds of dollars in ammo testing.
4.) Start right around 130 grains of Retumbo. Don't be concerned with velocity from the get-go.
5.) Base-ogive distance is of great concern to you. If you don't know it, you're swimming in the great "sea of the unknown"
6.) Don't mix jacketed and solid projectiles - even if it don't matter, it adds one more variable that may leave you confused at the end of the day
7.) Don't believe the "it will stabalize at longer distances" mumbo jumbo. If you want a 1/2 MOA gun, 100 yards is fine for testing just like it is for any other caliber. If it is not, you should be testing your other calibers beyond 100 yards too
8.) Clean your darned gun. Even though some folks don't believe in this, nobody is saying that their gun shoots better dirty, and it they do......well tell them you'll come dirty it for them
9.) Don't skimp on equipment. Factory ammo is over $6/round and reloads are upwards of $3 a round. "Penny wise and pound foolish."
10.) If you push the envelope on pressure/velocity, you may blow out your primer pockets and waste brass for 50 FPS which for a round like this may be negligible.
11.) Retumbo seems to be the "go-to" powder for this cartridge. Dogtown thinks RE-33 may be a great substitute. Temperature sensitivity may be an issue. But if you're going to add powder as a variable, you're going to blow a lot of money testing myriad different combinations
12.) Fed 215M primers are the only suitable primers
13.) Concentricity? Does it matter? I don't know, but again, at $3 a pop for reloads, do you want to take the chance? Maybe the same goes for neck thickness/tension, even though I ignored that
14.) Annealing? I don't know. But if your primer pockets are going to expand first, why bother with annealing?
15.) Don't waste your time with factory ammo unless you're going to just shoot the DTA stuff and be done with it.

 

[secondofangle]

Chrono'd the rounds today. the 132 grain Retumbo loaded to the lands (base to ogive 3.391) Fed 215M primer, new Jamison brass, have a muzzle velocity (red Chrony about 5 feet from muzzle) of 3045-3055. At barometric pressure of 24.23 mmHg and about 40 degrees and 85% humidity, with all the other variables (hopefully) entered correctly into Shooter, that gives me 13.9 MOA to get to 800 yards. I dialed in to 14 MOA and it was dead on the 10 inch 802 yard gong for several shots. In fact, we never missed that gong even with variable wind. This bullet don't mind the wind.

I think I have found my load. Not the hottest and fastest, but I will rest comfortably knowing that I'm less likely to have a catastrophic failure and hopefully my brass will last longer too....

 

[Colcalcipherol]

secondofangle, are you sure your base to ogive is 2.391"? The case itself is roughly 3.04" and the SMK is on the order of 1.770". Is it possible you made a typo?

 

[Colcalcipherol]

One other question: Are those ammo sleaves you have pictured above for the .50 cal? I need to find some sleeves like that.

And I agree with you about the Viersco dies--they are incredible! I have had a set for some time but finally put them to use last weekend.

 

[secondofangle]

You're right it's 3.391 I had to subtract for the hornady insert and I subtracted twice. Oops

They're the boxes that come with the DTA ammo and yeah they're 50 cal boxes. But MTM makes a case the blue one that fits the SMKs. Too. Probably wouldn't fit a solid

 

[secondofangle]

Tonight I loaded up about 40 rounds of the 132 grain loaded to the lands. I made a shell holder (see pics). I used a 3" wide and 11" long particle board with 5/8" holes drilled 1" apart. They're a bit slightly big but I didn't have a bit slightly smaller to drill the holes. I doubled the boards even though the cases would not tip with a single board, so they were more stable. I glued and nailed a thin piece of plywood to the bottom as a base. It works very well. Variations are possible but this is a starting point.

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[Shoottothrill]

Keep us posted enjoying the thread.

 

[Colcalcipherol]

Second,
Are you seeing any pressure signs at all? I am seeing extractor marks even on rounds loaded by CheyTac. One guy thusfar, although not a CheyTac user, suggested I get the weapon evaluated.

 

[secondofangle]

CHeytac loads are overpressure

Second,
Are you seeing any pressure signs at all? I am seeing extractor marks even on rounds loaded by CheyTac. One guy thusfar, although not a CheyTac user, suggested I get the weapon evaluated.

You can find a thread on this elsewhere, but I had 4 case ruptures with the Heytac factory ammo and I sent the rest of the lot to them and they admitted that it is too hot in chambers other than their M200 system and they were going to have to "drastically" reduce the load.

I think 132 of retumbo is as hot as you wanna go in an HTI with 350 SMKs if you're loading near the lands. At least it is in my gun. I had loaded as high as 136 but I think that was pushing it way too hot and for little return...