Sudden velocity drop

[Nick LD]

So I noticed a wierd happening for the first time while doing load development for my 45-70. Loading once fired starline brass with Hornady 350fp interlock and Imr 4198. Dispensed each charge on rcbs charge master and verified each charge on my dillon eliminator beam scale. I labeled each case with the charge weight after I charged it. Graduated charges by .5 grains from 34-43 grains, max according to book is 45.3. used Lee fcd to crimp at cannelure and each round was individually fed to eliminate the possibility of setback during devolpoment. Velocity was read with my beta chronny. The issue I had was at 43 grains I had a sudden drop of about 104 fps. 1778 fps at 42.5 gr and 1674 at 43 gr. Being how I'm positive the charge weights were correct what could possibly cause that. No signs of overpressure on case or primer.

Thanks in advance, first time I've come across this and my googlefu couldn't find anything about it on the interwebs

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[Rev]

How many rounds on the barrel? I went Through something similar recently and it was just a bad barrel blank, luckily we found it out pretty quick before I wasted a lot of components trying to make it shoot. Bad barrel is probally the least likely things but there’s a lot of things that can cause velocity drops. How long as it been since it was cleaned? Carbon deposits and build up can cause velocity jumps and drops because it directly effects the gasses/ pressures In the barrel. If it’s a newer barrel clean it a few times and try again. Also coulda just been a bad load, I’ve had those a few times. First thing would be to run a patch or two down it and load up and try again just to confirm. I’m not familiar with that particular chrono graph but I’d look at it too, maybe try to get another one to double check against. Hope it helps,
Rev.

[Nick LD]

How many rounds on the barrel? I went Through something similar recently and it was just a bad barrel blank, luckily we found it out pretty quick before I wasted a lot of components trying to make it shoot. Bad barrel is probally the least likely things but there’s a lot of things that can cause velocity drops. How long as it been since it was cleaned? Carbon deposits and build up can cause velocity jumps and drops because it directly effects the gasses/ pressures In the barrel. If it’s a newer barrel clean it a few times and try again. Also coulda just been a bad load, I’ve had those a few times. First thing would be to run a patch or two down it and load up and try again just to confirm. I’m not familiar with that particular chrono graph but I’d look at it too, maybe try to get another one to double check against. Hope it helps,
Rev.

It's a pretty new Marlin 1895 SBL. Have about 100 rounds down the tube. I clean it after I shoot it but admit that I probably could do a better job cleaning the barrel. I have been thinking about getting a new chronny, I get alot of errors.

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[Rev]

Working up loads like you are is a great way to find flat spot to load to, and sometimes when you get to a flat spot in the speed/ pressure you can see higher charges go slower than the one before it but thats usually no more than 5-10 FPS. It may verily be your chrono if it’s bad errors before. I’d suggest trying to find a magneto speed v3 or a lab radar to barrow.

P.s. I hate cleaning and am usually pretty lazy with it too lol.

[Nick LD]

Working up loads like you are is a great way to find flat spot to load to, and sometimes when you get to a flat spot in the speed/ pressure you can see higher charges go slower than the one before it but thats usually no more than 5-10 FPS. It may verily be your chrono if it’s bad errors before. I’d suggest trying to find a magneto speed v3 or a lab radar to barrow.

P.s. I hate cleaning and am usually pretty lazy with it too lol.

I've been drooling over that lab radar ever since I saw one being used at the range. It will be my next reloading purchase for sure. Probably once I sell my lock n load ap I'll get that.

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[sagela]

Nick, how many rounds did you chrono at each charge weight? If only one, try measuring 5 of them at one charge and note the spread (SD).
It could be your 100'/s drop is within the margin of error for that load/barrel/brass/bullet/charge/powder/temperature, etc.

+1 for the LabRadar. It's fantastic and you can get an ap for your phone to control it with. And you don't have to worry about hitting any sky screens.
Only problem I have had so far with ours is in measuring some Winchester generic .22-250 50 grainers. I blew a dozen rounds before I realized the bullet
was going faster than the LabRadar can READ! (3900'/s)

[JoeB]

If you've only loaded one round of each, I wouldn't worry too much about it. If you loaded 3-5 rounds at each weight, and the average of the 43gr load is 100fps less, then, stop. You might not have enough crimp. Is it a taper, or roll crimp FCD?
If its taper crimp, then, try using the roll crimp feature on your regular die. In a separate step.
I love the FCD for certain things, but, there's no substitute for a heavy roll crimp.

[Nick LD]

Thanks for all the replys, I loaded 2 rounds for each weight and both rounds were similar in velocity. I did use a Lee FCD crimp. I'm just now getting into loading for my big borrow guns so I've never had to roll crimp before. I think I'll stay in my flat spot I found around 1750 fps and try again with a roll crimp working up to the same load if you guys don't think it's something I need to worry about. When ever I see something out of the ordinary I usually stop and start over. This is the second load I've tried with this gun, had to scrape the first one

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[JoeB]

I dont think you have anything to worry about. Ive seen much, much hotter 45/70 loads.
Roll crimp can help produce extremely consistent straight wall rounds. Make sure you trim your all cases to the same length. I get much better results when I crimp in a separate step, with the seater plug removed.