Strange results with 7mmSTW

[stagpanther]

The weirdness continues

So I loaded up some test cartridges using 162 gr ELDx's driven by 77 grs of H1000. I shot two seperate 5 shot groups on two different days so as to slow down the erosion on the throat/free bore. I shot a couple of foulers prior to each 5 shot group since I thoroughly cleaned the bore between outings.

I also was careful to equalize the cases and bullets as best I could within the constraints of limited supply of both.

The wild spread of velocities continued with ES in the 80 to 100 + fps range, and on both outings the last 3 shots came together really well after the first two shots seemed to randomly impact. To make matters REALLY strange--the biggest velocity deviation shots are within the 3 shots that are touching (!).

I hereby dub this rifle the White Rabbit.

Shots are at 160 yds

 

[GeauxTide]

With the ES and SD numbers, I'd abandon that powder. Also, the recoil velocity of that round should get a rigid stock with full length aluminum rail and no thumbhole.

 

[stagpanther]

With the ES and SD numbers, I'd abandon that powder. Also, the recoil velocity of that round should get a rigid stock with full length aluminum rail and no thumbhole.

I already switched back to a MDT XRS stock--it's quite rigid and balances well. I've tried RL 33 and H1000 both of which have worked well in similar-sized big magnums requiring slow burning powders, but I have other powders I could use as well. I may switch primers and see if that makes any difference. I'm not seeing any evidence of late peak/burn at the muzzle. It seems to me the bore sorta "comes together" after a requisite amount of fouling--but I've never seen that in other match-grade Shilen barrels.

 

[scatterbrain]

Stagpanther, have you though about IMR 4350. It works great in my 300WM and 7mm Mag and most other rifles I load for that have slower powders recommend to use. There is published loads for 4350. Looking at your target I will ask you what your trigger pull is and how much flinch you are getting from the recoil. My gun of choice to shot is becoming a 222 Rem Model 700 vintage 1964.

 

[Unclenick]

Does sound like a powder ignition issue, but I don't have an answer for it off the top of my head for it. I can say several testers of primers have said Federal 215 Magnum primers are the most vigorous, so if you aren't using them already, I would try them next. I would also try a couple of primer seating strategies. Some have found lowest velocity spread when seating so the anvil feet just kiss the bottom of the primer pocket, while others have found they have to seat hard to get lowest SD, with the primer going 2-4 thousandths beyond the point of contact between the anvil feet and the floor of the primer pocket. You will likely have to experiment to find out what is actually best with your primers, cases and your firing pin.

The throat heat cracks are caused by the fact the surface temperature of the bore only penetrates a couple of thousandths of an inch during the time the bullet is in the barrel and the temperature and pressure are high. This produces a temperature differential between the bore surface and the steel underneath it. This causes the heat stress cracking. You would, ironically, have less temperature differential if you didn't let the barrel cool so much between shots, but neither do you want to refire so soon that the peak temperature reached inside the barrel got too high. There is probably an optimum rate of fire as far as barrel and throat life is concerned, but I don't know what it is.

 

[stagpanther]

Stagpanther, have you though about IMR 4350

I think that might work for smaller bullets, perhaps, but it's probably a bit on the fast side for the heavier ones that I use--162 gr to 190 gr. H4350 is one my favorite go-to powders for medium sized cartridges.

Federal 215 Magnum primers are the most vigorous, so if you aren't using them already, I would try them next.

The fed 215's have always been my LRM primers of choice--I haven't seen them for sale in years. It's only with the release of White River's LRM primers that I've started shooting my big magnums again. I had my first failure to fire of the WR primers today--so that was one out of the first 150 I used. Anyway--I have some proven very accurate 338 LM loads that I've previously developed, so I think what I'm going to do is try one or two of them out with the WR primers and see if they make any difference with them. I do so love shooting that rifle.

 

[stagpanther]

So I shot a 4 shot group at 164 yds (a 5th shot I needed to bore sight the scope) and the stats were OK, around ES 20 fps and SD 10 fps--but still about twice the figures as what I had previously recorded using the fed 215 primers. I've shot better groups at 200 yds with the same load; though the thick fog may have had some effect. While the primers may have been slightly off compared to the fed 215's--I don't think that by itself would account for the big swings in my 7mm STW loads.

 

[stagpanther]

I've had a bit of time to take yet another look at the 7mm STW--I've found sometimes it's best to step away from a problem and "cool down" before re-engaging it. Gave some thought to the many useful suggestions as well, so I started "sleuthing" to see what I could find. I also put the action back into the Boyds stock, just because I really would like to get it to work well if possible.

Several things popped up that grabbed my attention; the first was the barrel was cocked slightly to one side in the barrel channel--I'd never noticed that with other actions mounted in the stock--so something some place was causing asymmetry when the action screws were tightened. I then noticed the new bottom metal was not entirely level in the inlet, so I figure I'm going to need to grind it down further and then bed the bottom metal for a good tight fit.

The wide variations in ES and SD though is more specific to the cartridge and what happens when chambered. Evidence I'm seeing in both the case mouth step in the chamber as well as gas leakage down the neck leads me to believe I'm not getting a good seal of the neck to the chamber as the cartridge fires. I've decided that instead of doing a resize and headspace off the belt I'm going to let the shoulder come forward after firings and instead try to headspace off the shoulder. In the process my hope is to find the exact length of case and neck that works best to provide the best seal.

 

[stagpanther]

OK--I've worked the inlets so I have a nice fit for the bottom metal and the action seems to seat nice and solid in the stock--though I haven't bedded the bottom metal yet.

I remain stranded in woo-woo land on this one, I've shot 4 follow up groups and they all behaved in the same way. the first two or three shots strayed randomly but the last 3 shots always come together for a nice tight group.

I shot one group with a totally clean bore, one with some fouling, one spacing the shots 4 minutes apart and one shooting fairly rapidly. Same result every time.

My impression is that for some reason the performance picks up once the barrel gets quite hot--not a good thing since that also means the throat is wearing fast. My conclusion is that somehow, something, needs to swell up in heating in order to get consistency.

 

[georgehwbush]

that sounds like the place for cryotreeting, but i'm not trying to spend your money.

 

[stagpanther]

I knew when I bought the barrel that I would most likely burn it out just finding some good loads--cryo will certainly be considered next time around (if there is one). The barrel is plenty long--so as long as enough material is left I've thought about reaming it again before the fire-cracking gets too advanced.