Difference velocity first shot

Moderators: gds, bakerjw, renegade

Post Reply
sniperd46
New Member
Posts: 9
Joined: Thu Mar 19, 2020 10:55 am

Difference velocity first shot

Post by sniperd46 »

Hi,

I have a 300blk Ruger, I found a very accuarate load with a hornady 208 eld-m bullet and 10.9grs of n120, lapua case, federal gm205 primer oal 61,50mm.

My problem is that the first and always the first shot (suppressed) is always faster than the others.
Example:
1 shot: 308 m / s
2 shots: 295 m / s
3 shots: 294 m / s

I use magnetospeed V2

This behavior always happens, on different days.
Can you help me?
BlogSarge
Silent But Deadly
Posts: 1336
Joined: Mon Oct 21, 2013 11:10 pm

Re: Difference velocity first shot

Post by BlogSarge »

Interesting question. And one I have no answer for, but am curious and hope others chime in. Does it happen without a can on?

Maybe related to first round pop? A little extra O2 in the can v followup shots?

I’ve heard of ‘pre-loading’ the can with a shot of CO2 to reduce FRP. Maybe try that and see if it reduces the difference.
User avatar
dellet
Silent But Deadly
Posts: 6967
Joined: Fri Nov 30, 2012 1:25 pm

Re: Difference velocity first shot

Post by dellet »

As blogsarge said, check the velocity without the suppressor to see what the load itself is doing.
300 Blackout, not just for sub-sonics.
Blackout!
Silent Operator
Posts: 72
Joined: Sun Dec 14, 2014 6:45 pm

Re: Difference velocity first shot

Post by Blackout! »

Is the first shot from a clean, oiled barrel?
ReadyAimDuck
Silent But Deadly
Posts: 174
Joined: Tue Jul 14, 2020 11:08 am

Re: Difference velocity first shot

Post by ReadyAimDuck »

I know the speed of sound is lower at colder temperatures. Could this have anything to do with what is happening? I understand the temperature in the air outside is going to be relatively the same between the 1st and 5th shot, for instance. But strangely enough, I just went outside to send a few of my subs into the hill and the first one had a supersonic crack for sure. The remaining 7 or so of the same load had no supersonic crack. I am using a suppressor. Rifle barrel has not been cleaned and lubed in 50 or so rounds. Outside temp is in the mid 20s (F). No chrono.
BJK
Silent But Deadly
Posts: 355
Joined: Thu Jun 18, 2020 8:30 am

Re: Difference velocity first shot

Post by BJK »

A shot in the dark... (well almost, another hour :D )

Are you certain that it's not first round pop? Some (many, all?) cans do that until the O2 in the can is displaced by spent powder gasses.

It's by no means a definitive test, but fire another with a can with powder gas absent from inside and see if you get the noise. Then take off the can and blow some compressed air into it to get rid of the gas, then try it again and see if it happens with round #2.
ReadyAimDuck
Silent But Deadly
Posts: 174
Joined: Tue Jul 14, 2020 11:08 am

Re: Difference velocity first shot

Post by ReadyAimDuck »

I did not know about the pop associated with the first round through the can. It very well may be that. I have no chrono. I will step aside now, so as not to detract from what might be happening with the OP, who is actually testing via chrono.
rb288
Senior Silent Operator
Posts: 141
Joined: Tue Jan 20, 2015 8:07 pm
Location: Grovertown, Indiana

Re: Difference velocity first shot

Post by rb288 »

I will throw this out for consideration only, as, I'm just "guessing".

First shot with a cold barrel, then as the barrel heats up, the velocity stabilizes.

Just a thought.
I may have been on the losing side, but, I'm not convinced it was the wrong side.
User avatar
plant.one
Silent But Deadly
Posts: 6823
Joined: Tue Jun 23, 2009 8:31 pm
Location: Oakland County, MI

Re: Difference velocity first shot

Post by plant.one »

rb288 wrote: Thu Dec 31, 2020 10:29 am I will throw this out for consideration only, as, I'm just "guessing".

First shot with a cold barrel, then as the barrel heats up, the velocity stabilizes.

Just a thought.
usually they stabilize in the other direction though - up not down - and we would more expect the first round to be just a bit slower than following rounds. in this instance the first round is the highest velocity shot, the exact opposite as you'd expect.
Reloading info shared is based on experiences w/ my guns. Be safe and work up your loads from published data. Web data may not be accurate/safe.
This disclaimer will self destruct in 10 seconds.
Post Reply

Who is online

Users browsing this forum: No registered users and 17 guests