Thread: Funky speeds - Riddle me this

Hi team,

I have a load that is grouping well enough- Barnes TTSX 120g with H335 - 5 shots at 100yd (0.7") and 5 shots at 310yd (3") in my 7mm08 bushpig. 300yd is roughly my limit. I've measured drop and re-calculated velocity to be about 2940 fps plus or minus.

I have also measured velocity on two separate days with two separate chronographs. Each time the first shot read about 2920, which should be about right, and the following shots varied between 2770 and 2900. I checked the chrono with my 7mm Rem Mag and it was very consistent and gave roughly the expected velocities.

Has anyone encountered funky readings like that with certain loads before? I trust the groups and the drop over the chrono, but it was interesting that it happened on separate days with different chronos.

Cheers,
Elliott

Are you using a suppressor? What's the extreme spread like over 5 shots without it?

I've seen the first shot through a suppressor come out at higher velocity. But 2770-2900 is quite a high extreme spread anyway and I'd be trying a different powder to bring that down.

Originally Posted by Pommy

Are you using a suppressor? What's the extreme spread like over 5 shots without it?

I've seen the first shot through a suppressor come out at higher velocity. But 2770-2900 is quite a high extreme spread anyway and I'd be trying a different powder to bring that down.

Yes to the suppressor - have not shot without it though.

It is a high ES; however, I'm leaning towards not trusting the readings because the groups are good. If the velocities really were that different I would not be expecting to get a MOA group at 310yd? Or maybe it would? I'm not sure.

But your 7RM was nice and consistent during the same sessions over the same chrono?

If 300's your limit and it groups well out to 300, so what right?

You could chuck highest and lowest velocity shots into a ballistic calculator and, assuming your POI at 100 is the same, you'll know how much the ES causes vertical dispersion at longer ranges.

What are you weighting your loads with - maybe you have the odd "funny" round that you've just happened to pick up with the chrony at 100M (did you chrony all shots ??). Bit of a mystery alright cause if you were only using 3 shot groups I see a bit of what you've reported (good groups with shit ES) but its a lot less likely with 5 shot groups

Originally Posted by Tentman

What are you weighting your loads with - maybe you have the odd "funny" round that you've just happened to pick up with the chrony at 100M (did you chrony all shots ??). Bit of a mystery alright cause if you were only using 3 shot groups I see a bit of what you've reported (good groups with shit ES) but its a lot less likely with 5 shot groups

Yeah it's got me confused. Weighing with Hornady beam scales - they're pretty good.

I am wondering whether it could be a genuine large ES, as @Pommy suggests - I just ran the data and there is only 1" or so vertical spread at 300yd. So that could be within the 3" group, and potentially could be a lot tighter if the spread is reduced.

You can have loads that vary by 150-200fps and still shoot awesome groups at normal zeroing ranges.
There will be something amiss in your preparation to be getting a high ES. It is rarely powder.
Start looking at headspace, neck tension and even change primers.

Sent from my SM-T510 using Tapatalk

Originally Posted by R93

You can have loads that vary by 150-200fps and still shoot awesome groups at normal zeroing ranges.
There will be something amiss in your preparation to be getting a high ES. It is rarely powder.
Start looking at headspace, neck tension and even change primers.

Sent from my SM-T510 using Tapatalk

One thing I had noticed is that the powder (H335) sometimes sticks to the inside of the neck, instead of it dropping down.

I'm usually pretty careful about it but could miss the odd one.

Any tips on what I should be looking for to ensure consistency with headspace and neck tension? I'm pretty new to this, sorry.

Originally Posted by Edunn

One thing I had noticed is that the powder (H335) sometimes sticks to the inside of the neck, instead of it dropping down.

I'm usually pretty careful about it but could miss the odd one.

Any tips on what I should be looking for to ensure consistency with headspace and neck tension? I'm pretty new to this, sorry.

Set your dies up for minimal shoulder bump. Don't mix headstamps. Keep them in batches that have had the same number of firings. Clean the lube out of the inside of the cases after you've resized them (even OneShot that claims not to contaminate) and trim them all to the same length. The headspace comparator kit will show you how consistent your base to shoulder measurement is. If you're using a regular FL die then neck tension is what it is - you need a bushing die (and high quality / neck turned brass) - or a collet die - to start playing with neck tension. Annealing helps too if that's an option for you.

Originally Posted by Pommy

Set your dies up for minimal shoulder bump. Don't mix headstamps. Keep them in batches that have had the same number of firings. Clean the lube out of the inside of the cases after you've resized them (even OneShot that claims not to contaminate) and trim them all to the same length. The headspace comparator kit will show you how consistent your base to shoulder measurement is. If you're using a regular FL die then neck tension is what it is - you need a bushing die (and high quality / neck turned brass) - or a collet die - to start playing with neck tension. Annealing helps too if that's an option for you.

Great info, thanks!

I'm always using the same brass (same firings too). Could do a better job of cleaning lube out. Always trim to same length. I'm not sure how to set up the die for shoulder bump, but will get on the google for that.

I am using a regular full length sizing die for both my 7mm rem mag and 7mm08. Kind of hesitant to fork out for another die without trying the other stuff first.

I'm keen to get things as precise as possible, but the reality is it will be used for hunting once I'm set up - not chucking pills at steal at crazy distances. If I can reduce the ES without having to buy too much gear that will be awesome.

Pretty much what pommy said

Sent from my SM-G975F using Tapatalk

What type of chronograph are you using @Edunn ?

@Edunn how full are your cases with the H335? With my 308 i found there was quite a bit of empty space in the case. Had high ES (not 200fps though). Have moved to 2206H and seems to have a better case fill. Groups tightened up even more yet to measure MV however. Another thing I found was the H335 had pretty big swings in pressure with temperature, 2206H is much better in this regard.

How many firing has your brass had?
could be neck tension (but 130 fps difference is a lot), time to anneal or get new brass maybe

Originally Posted by Hunty1

@Edunn how full are your cases with the H335? With my 308 i found there was quite a bit of empty space in the case. Had high ES (not 200fps though). Have moved to 2206H and seems to have a better case fill. Groups tightened up even more yet to measure MV however. Another thing I found was the H335 had pretty big swings in pressure with temperature, 2206H is much better in this regard.

This is good to hear @Hunty1 - I have some AR2206H left over that I was thinking about trying. The cases seem pretty empty, which I was worried about too. I find 2206H much nicer to use but did not get great grouping when I tried. Will do a proper ladder with it, though, and see how I get on.
@zimmer the first test I did was with the ProChrono and the second was with the F-1 Chrony (the SI forum one). Both gave similar results.