Max effort rifle build steps 375CT

[doctahjones]

I want to build my next rifle as well, accurately as possible. From my research this would include ordering a custom reamer and maybe a custom full length sizing die. I've always just used the gunsmiths reamer and bought off the shelf dies, so this is new to me.

Do these steps make sense?
1) Order brass and bullets.
2) Expand, Neck trim, just enough to clean up necks, about .001" in my experience.
3) Uniform brass length + chamfer, I realize this is typically done later, but if you are ordering a custom reamer, why not do it now?
4) Seat a bullet long (single shot.) I can use quickload to get an idea of how it will perform since I have Ba values for the powder I'll be using.
5) Send the loaded uncharged round to PTG and have them make a custom reamer based on my case dimensions, neck diameter, neck length, suggested bullet jump from the manufacturer.
6) Send the reamer, action, barrel to my gunsmith.
7) Order a custom full length sizer die (Warner) -or- buy a Forster full length sizer die honed to my neck diameter. If my reamer is close to saami spec I could probably get away with the Forster. If I'm not overworking my brass and concentricity is good, stick with it.

Of course the nice thing about doing it the long way is that future barrels will have the same chamber. Brass should be minimally worked, and concentricity should be excellent.

Questions:
- Do these steps look good?
- What clearance should I specify for neck diameter and length ( I think .005" on diameter and .01" on length, but what to be sure.)

Thank you.

 

[SKWERLZ]

Don’t try to get too tight with the reamer at the base.

otherwise it’s all good.

 

[ShaunBrady]

You didn't list what you intend to do with the gun. From the cartridge, I'm going to guess it's not 100 yard bench rest, maybe something along the lines of ELR.

The popular misconception is that ELR is a precision game and that bench rest accuracy is required. It is not and chasing it burns valuable barrel life at 100 yards that could be used to better effect at longer distances.

This is the error budget for a well tuned nominal 375CT. 390 ATip, 2950 fps, 0.5 moa 10 shot group, 15 fps velocity ES, 1.5% BC spread.



The 3 components are Velocity spread, BC variation, and precision. They're not simply added together for the worst case, a RMS calc is used for the estimate. Each term is squared, the squares are added together, and then the square root of the sum is the last operation. It's pretty easy to spot check this with a solver. The charts are produced by exporting range cards from AB Analytics and dropping them into Excel.

If you play with that calc a bit, it'll become apparent that even with very low values for velocity and BC spreads, the precision really doesn't matter past a mile. That is, 1/2 moa, 1 moa, even 2 moa won't change much. If we consider that a custom barreled gun shooting premium bullets within 15 fps isn't very likely to put much over 1 moa no matter how long the string is, we might conclude that the precision takes care of itself if the rest of the house is in order.

That's about as good as it gets, 15 fps / 1.5%, let's call that "expert" level. A beginer might start at about 4x those values and we can define intermediate as twice the expert values.



The other popular misconception is it's all about velocity. Velocity moves those curves back and forth on the horizontal axis by about 75 yards per 100 fps. There is far more to be gained by dropping down to the next curve than the next 100 fps. Velocity is good, right up to the point it starts costing velocity or bc spread. In the second half of the barrel's life, it starts contributing to the BC spread and after seeing the effect first hand, you'll be replacing barrels at lower and lower round counts. That next 100 fps costs 1/4 to 1/3 of the current barrel life. If you want max effort "expert" level gun, this is one of the places it comes from. Figure no more than 600 rounds of peak performance for a 375CT that shoots 10-15 shot strings.

I've included a wind level in this one to start reinforcing the idea that range time is much better spent at distance than 100 yards. 2 mph of wind means plus or minus 1 mph. Over the next mile or two. From ground level to max ordinate, say 200'.

This is the background for the comments to follow.

 

[ShaunBrady]

So....

1. Good plan. It's pretty much Peterson brass. Bullets are 400 Lazer or 390 ATip. Either will give a solid baseline to test others against after you've bootstrapped yourself. It's either solid or jacketed for the barrel twist and reamer, trying to do both will do neither as well as it could.

2. That approach assumes the next lot of brass will be the same. It may not. I'd work to a neck thickness maybe 0.002" smaller than the nominal for that lot. That'll give some wiggle room after you've hard wired your reamer and sizing die decisions. If the bearing surface is into the shoulder, focusing on the neck shoulder junction when turning gave the lowest velocity spreads. The biggest gain from moving the bullet into the neck was not having to freshen the necks up every 3-4 firings. Use a turning tool that has a boring mandrel. One of the sources of velocity spread is leakage past the bullet as everything pressures up. Look inside the neck after you've run a sizing mandrel through it. Those streaks are high spots, gas will leak in varying amounts through the low spots.

3. I'd square the sample piece and chamfer it but I'd work to the drawing for deciding on reamer length.

4. Seat a jacketed bullet so the bullet bearing surface is 25% of the caliber ahead of the nominal neck shoulder junction. This is the easiest way to maintain velocity uniformity. With a solid, the driving band needs to be at least 0.010" ahead of the case mouth at max case length. That or the bearing surface may set the minimum length. I use 0.025" jump with a new barrel and use the same cartridge OAL for the life of the barrel. That philosophy also gives some wiggle room if you change bullets.

Ba is only the start of a QL model and I generally leave it alone. The 375CT is magnum enough that the weighing value needs to be adjusted. I don't remember what the default is, I use 0.4. That's based on testing I did varying the overbore ratio on 338 Lapua cases. Using bits of other people's modeling technique is sketchy though. The weighing factor has a couple explanations in the manual, but it's basically the knob that adjusts the pressure down and velocity up when you reduce its value. The other knob I turn is the start pressure. I'll adjust it to match observed velocities. With ELR cartridges, it increases significantly with the age of the barrel and is one of the things I use to gage retirement. To make any of that work, the ammo temperature and moisture content of the powder will need to be tracked and their effects quantified.

5. Nobody who's ordered more than 3 reamers in the last 10 years still uses PTG. I've had good luck with Manson. Others will say the same about JGS.

6. You might want to not reinvent the wheel and just go with an experienced gunsmith and his reamer. Do get the drawing. I do reamers much differently than I did 10 years ago. Some of it is changes in design philosophy, some of it is new products.

7. Dies are a sticking point for 375CT. This would be another good place to work with an experienced 375CT smith. Avoid anything based on a standard 7/8x14 body. I go with 0.006" neck clearance. The sizer brings it down to 0.004" under final, the mandrel back to 0.002" under final. The neck is worked 0.010" every time. That's enough to introduce some runout but with a 1 piece die, not many are more than 0.002" tir and none are over 0.003" tir. 0.003" tir has been shown many times to be a third order contributor to accuracy and accuracy is a second order problem in ELR.

You may need to compromise on the neck thickness to make the 0.004" under work for an available die.

I use the AMP annealer to bring the brass back to the same hardness every time. Saves resetting dies. My experience has been trying to avoid "over working" brass is more trouble than it's worth. I've had good results with torch based annealers over the years including the BenchSource. I tended to under anneal judging from the AMP results. Ignore anything on annealing that mentions 750 degrees.