Little Help - 338LM Noob

[LawnMM]

So I started doing some load development for my 338LM and I'm seeing some oddities during the process. Its a 1:9.4 twist, 24" barrel. On the upside, it seems very pressure insensitive. I've been working on a load for Hornady 285 HPBTs and book max is 93.0gr of Retumbo at around 2750. On my first trip to 93.0gr I was seeing averages of about 2795 so that gave me high hopes for making 2850 or better with these bullets. However, I went from 93-94gr and actually lost speed?! Winding up around 2735ish. The case is quite full but not compressed so I figured perhaps this was a case of the cartridge being overly stuffed and stifling the ignition inside the case. I tried 93.0gr again and again found myself down at around 2735fps.

Questions:
Did your barrel take some time to settle in? My 6.5 Creedmoor picked up speed till around the 100-150rd mark, this barrel so far has around 50 through it.

Any other ideas why I'd be seeing almost 2800 at 93, 2730-2760ish up to 94, and now 2730 at 93?

I've got a nice supply of these bullets so I was hoping to make them work but if I can't get 2800 out of them I may sell what I have and go with some Berger 250s instead. I suppose I could load up again from 94-95gr and see if that puts enough steam behind them to get them up there but I don't know if there's going to be room in the case for that and I'm also realistic. If the barrel isn't long enough to get these up to 2800 even with the case topped off then maybe I need to re-evaluate the projectile choice and sell the lot that I have.

Anybody looking for some Hornady 285s? I'm sitting on about 600 of them, same lot number.

 

[.50 Cal Cow]

What chrono are you using?

Was there a change in weather between the first trip and second trip?

Was the brass virgin, once or multiple times fired brass the first time out?

Was the brass virgin, once or multiple times fired the second time out?

Did you change the OAL length of the load between first trip and second trip?

 

[.50 Cal Cow]

I might be wrong but iirc I've read also that diminishing to decreasing velocity as charge weight goes up may be an indication of an over max load.

And the 338 LM may not always be obvious when showing pressure.

 

[LawnMM]

What chrono are you using?

Was there a change in weather between the first trip and second trip?

Was the brass virgin, once or multiple times fired brass the first time out?

Was the brass virgin, once or multiple times fired the second time out?

Did you change the OAL length of the load between first trip and second trip?

Magnetospeed Chronograph

If there was 5F change in temp I'd be shocked.

Virgin Lapua Brass on all range trips

Same OAL each trip

 

[LawnMM]

I might be wrong but iirc I've read also that diminishing to decreasing velocity as charge weight goes up may be an indication of an over max load.

And the 338 LM may not always be obvious when showing pressure.

That's the first I've heard of the 338 not following the rules that govern every other cartridge. I'll clarify though, there is ZERO sign of pressure. The bolt cycles freely without any binding or sticking. There are no ejector stamps or marks at all. The primers are round and unflattened. There is no cratering where the pin struck any of the primers.

I do believe the burn rate of Retumbo may be a factor. I have had powder fill a case without pressure before so I'm thinking I've hit the upper limit of what Retumbo can produce from this gun. I probably need to switch powders or projectiles and I'd rather switch projectiles than go on the hunt for different powder.

I am interested in seeing if anyone else has experienced their barrel picking up speed as it breaks in though

 

[.50 Cal Cow]

That's the first I've heard of the 338 not following the rules that govern every other cartridge

Read it a couple times somewhere but never really understood why it was said since I try not to push the envelope with the 338 too much. I have only experienced a slightly craterered primer and lightly swiped ejector mark a time or two.

Anyways don't have your answer. I do have a box and some loose 250gr berger elite hunter if you wanted to try PM me and we can work something out.

 

[LawnMM]

Thanks for the offer, I might check the LGS first

 

[Xtremegunnr]

I have found in my 338 that I top our or loose velocity like you have seen with very high loads. That being said...I load 99 grains of retumbo under a 254 (3100 fps) at just under 4" oal and 95 under 300 grain bergers )2875) at roughly the same oal. 26" DTA A1

Any more and I had worse groups and no increase in velocity

 

[LawnMM]

I'll post back later in a day or two on results, I looked at Hodgdon's data again and my own off my chrono. 94gr is max under a 300gr projo per Hodgdon and that's loaded at 3.6". I'm loading to 3.715 and using a bullet 15gr lighter so I'm going to see if going from 94-95gr gets me where I want to be with velocity. I've run into zero signs of any pressure so far and I'm making small .2gr jumps so if I start bumping into pressure I'll just stop and re-evaluate my projectile choice. I've got a bunch of these Hornadys though and everything I've read says they rock for ELR so I'm going to see if another few tenths of a grain will get me where I want to be with these. I'm only about 40fps shy of the bottom end of where I'd like to be, another .2-.4gr might fix that.

 

[LawnMM]

So, success, I haven't finalized it yet but 94.4gr gets my average up to around 2797 or something like that with a 9fps SD, I'm going to try a larger sample size over the chrono and try 94.3 and 94.5 and see what looks best. Its accurate at this node though I knocked the red off the bull on about 6 out of 8 corners of somebody else's shoot n c's that were still hanging there. Should have this about finalized by Thursday

 

[bruddah]

Tag

 

[Xtremegunnr]

Glad to see you found a sweet shooting load. Even at those velocities, that bullet is going to be effective far longer than most people can shoot

 

[LawnMM]

Settled on 94.4gr of Retumbo, that's over max so please don't try it yourself without working up. I'm getting 2799fps Average with the 285gr Hornady BTHPs which my handy AB app says should be supersonic to a little past 2000yds which is about the most I plan to ask of this rifle anyway. If I fall in love with ELR and I feel like the 338LM is holding me back I will build up a 375CT.

For now I'm excited to try some of the boundary breaking shots, 1500,a Mile, etc. Getting 9fps SD so it's already pretty consistent, I might goof with seating depth a little and see if I can tighten it up a tad more.

 

[lash]

I'm following your journey, as I plan on following the same path starting later this fall. Fixing up a salvage 110 right now for that purpose.

 

[LawnMM]

Next up I'll see how it groups at 100/600/1000,probably next week on that