First shot in a series always has higher MV

[skr]

I’m fairly new to handloading and I’ve got two problems I can’t seem to crack. I’m working up a new load and I’m seeing high SD and the first shot in each series has noticeable higher muzzle velocity than the others. The "first shot is fast" phenomenon is across about 19 series of 5 shots in two different rifles. Is this to be expected? Any idea what the cause is? Barrel cooling while I reload? Any recommended remedy to either problem?

I’m seeing fairly high SD of 20-30 fps at the current stage in working up a load. I checked my data from another rifle that I worked up a decent SD (~15 fps) load and those all had the same first shot is higher mystery going on.

This is when working up a 7mm Remington Magnum load in a newish rifle (100-200 rounds now). 180 gr Bergers, Retumbo, virgin Norma brass and Federal 210M primers.

I run it in a neck expander mandrel in order to neck turn. I conform the primer pocket, trim to length, chamfer/deburr. In this case I did not weight sort. I have not yet reloaded used cases so no change in neck tension yet.

My process has been as follows: ladder test 0.1 gr difference from 68.2 gr to 69.2 gr. Did 5 shot series of 68.3, 68.4 and 68.5. Picked 68.4 due to smallest group avg to center and SD (not counting first shot). Did a seating depth test and -2mm from the lands seems to do best (again not counting first shot).

Below you can see example velocities when testing seating depth. Depth in mm from start which was bullet jammed into the lands. Velocity in fps.

Velocity is measured using a Magnetospeed with no changes between series. Each series is shot with a shorter or longer break between (either ~5 minutes for reloading and whatnot, or more like 10-15 if there’s a target change period at the range). To kick off the day I shot a ~10 round string of factory HSM ammo, this got a 22.6 fps SD. No cleaning between each series.

Relative seating depth (mm) Velocity (fps)
0 2945
0 2864
0 2891
0 2871
0 2866

-0.25 2922
-0.25 2882
-0.25 2866
-0.25 2879
-0.25 2873

-1 2921
-1 2849
-1 2863
-1 2840
-1 2866

-2 2906
-2 2857
-2 2842
-2 2860
-2 2842

-3 2879
-3 2838
-3 2850
-3 2848
-3 2874

 

[Cool Tool]

I’m fairly new to handloading and I’ve got two problems I can’t seem to crack. I’m working up a new load and I’m seeing high SD and the first shot in each series has noticeable higher muzzle velocity than the others. The "first shot is fast" phenomenon is across about 19 series of 5 shots in two different rifles. Is this to be expected? Any idea what the cause is? Barrel cooling while I reload? Any recommended remedy to either problem?

I’m seeing fairly high SD of 20-30 fps at the current stage in working up a load. I checked my data from another rifle that I worked up a decent SD (~15 fps) load and those all had the same first shot is higher mystery going on.

This is when working up a 7mm Remington Magnum load in a newish rifle (100-200 rounds now). 180 gr Bergers, Retumbo, virgin Norma brass and Federal 210M primers.

I run it in a neck expander mandrel in order to neck turn. I conform the primer pocket, trim to length, chamfer/deburr. In this case I did not weight sort. I have not yet reloaded used cases so no change in neck tension yet.

My process has been as follows: ladder test 0.1 gr difference from 68.2 gr to 69.2 gr. Did 5 shot series of 68.3, 68.4 and 68.5. Picked 68.4 due to smallest group avg to center and SD (not counting first shot). Did a seating depth test and -2mm from the lands seems to do best (again not counting first shot).

Below you can see example velocities when testing seating depth. Depth in mm from start which was bullet jammed into the lands. Velocity in fps.

Velocity is measured using a Magnetospeed with no changes between series. Each series is shot with a shorter or longer break between (either ~5 minutes for reloading and whatnot, or more like 10-15 if there’s a target change period at the range). To kick off the day I shot a ~10 round string of factory HSM ammo, this got a 22.6 fps SD. No cleaning between each series.

Relative seating depth (mm) Velocity (fps)
0 2945
0 2864
0 2891
0 2871
0 2866

-0.25 2922
-0.25 2882
-0.25 2866
-0.25 2879
-0.25 2873

-1 2921
-1 2849
-1 2863
-1 2840
-1 2866

-2 2906
-2 2857
-2 2842
-2 2860
-2 2842

-3 2879
-3 2838
-3 2850
-3 2848
-3 2874

If you are setting at bench trying a load walk off an set back down 90% of the time first round is almost always faster than rest of the group.

 

[LCazador]

I'd see if shooting over a different brand chronograph would change anything before I'd get too perplexed about my loads. My experience with the Magnetospeed was not good. When tested against my Oehler it was all over the place.

 

[skr]

Thanks, I'll see if I can find another chronograph to borrow.

 

[jcampbellsmith]

First shot velocity discrepancies are a feature of your barrel and how it handles cold bore (clean or dirty) shots. I generally foul/warm my barrel before testing individual loads. One rifle I have consistently shoots first shot slower.

Regards JCS

 

[Ned Ludd]

The chances are that nothing is wrong with your chronograph. The MagnetoSpeed is one of the most reliable chronographs made. My suggestion would be to try not waiting 5 min between the first and second string, as an example, and see if you still get the high value for the first shot of the second string. My guess is that you won't. Another alternative is to simply shoot 6 rounds and use the last five shot velocities only. Although I can't say exactly what the exact cause is from the information you provided, I suspect there is something specific about your routine that is causing the high value on the first shot once you get back behind the rifle, not the chronograph.

FWIW - after shooting a 3-4 foulers, I can usually shoot 2 x 5-shot groups on a one minute cadence without the barrel heating up too badly. It will be just barely warm to the touch at the end, at which point I will give it 5 minutes or so before doing 2 x 5-shots more, etc. Obviously you don't want the barrel very hot, but it also doesn't need to cool down all the way to ambient temperature either for most people's testing purposes.

 

[Webster]

I’m fairly new to handloading and I’ve got two problems I can’t seem to crack. I’m working up a new load and I’m seeing high SD and the first shot in each series has noticeable higher muzzle velocity than the others. The "first shot is fast" phenomenon is across about 19 series of 5 shots in two different rifles. Is this to be expected? Any idea what the cause is? Barrel cooling while I reload? Any recommended remedy to either problem?

7mm Rem Mag a hunting rifle. It doesn't matter what the SD is if you get groups that are good enough for the distance you shoot at. You are not trying to make a bench rest quality rifle.

 

[jlow]

I guess I should ask - does the OP clean his barrel between strings?

 

[RonAKA]

I guess I should ask - does the OP clean his barrel between strings?

I wondered that too. However on a close check he did say: "No cleaning between each series."

 

[gotcha]

Thanks, I'll see if I can find another chronograph to borrow.

This should prove to be very interesting. ....... Leave the Magneto spd. on rifle and shoot through the sky screens and compare. I've seen this first shot faster phenomenon on my Magneto spd. and usually delete my 1st shot and average the rest. I've never had the opportunity to shoot with 2 crono's as outlined above. Let us know what happens.

 

[skr]

Thanks for all the responses. To answer one question: I did not clean between each string.
I just realised I did have some data on factory ammo in this particular rifle (same setup, magnetospeed and shot at the same time as some of the other data collected). They do not show this particular pattern, so it might be something I screw up during loading.
What I'll try: Test against a different chronograph. Shoot each series with a random order. Cut down time between series. Or just add a 6th throwaway round.

Thanks again. If I figure it out I'll report back

 

[jlow]

Thanks for all the responses. To answer one question: I did not clean between each string.
I just realised I did have some data on factory ammo in this particular rifle (same setup, magnetospeed and shot at the same time as some of the other data collected). They do not show this particular pattern, so it might be something I screw up during loading.
What I'll try: Test against a different chronograph. Shoot each series with a random order. Cut down time between series. Or just add a 6th throwaway round.

Thanks again. If I figure it out I'll report back

Thanks for the clarification. Testing using factory ammo is a great idea. So you lube your case or bullets in some way?

If I have say two or more series of reloads I am testing against each other, I always shoot them round Robin style i.e. shoot the first round from each series first, then come back and shoot the second round of each series, and so on.

This method avoids the rounds of a different series from facing a dirtier bore, or hotter barrel, more tired shooter, more wind coming up later in the day, etc. It's generally good practice to do it this way.

 

[skr]

I have not yet confirmed this but I suspect it's due to the first round being chambered for longer and heating up. I usually chamber the first one before I sit down and get comfortable. Second one I'm already on target so time between cycling the bolt and firing is much shorter. Doesn't explain why the data I had for factory is affected less though.
I have a second chronograph that I'll be trying out soon. just to make sure.

 

[Brians356]

I have not yet confirmed this but I suspect it's due to the first round being chambered for longer and heating up. I usually chamber the first one before I sit down and get comfortable. Second one I'm already on target so time between cycling the bolt and firing is much shorter. Doesn't explain why the data I had for factory is affected less though.
I have a second chronograph that I'll be trying out soon. just to make sure.

If you are chambering into a hot gun, that makes sense. Best to have them all bake in the oven, so to say, for the same time.

Conversely, chambering into a particularly cold gun (in winter) can cause the first shot velocity to be much lower than the following shots. In cold weather, I always stage the groups I am about to fire in a couple of warm pockets (shirt pockets, against my skin, under an outer coat) to warm them up for some minutes before shooting them. But a really cold gun will quickly chill the first round chambered, and you don't want to rush that first shot (or any shot), so a better approach is probably to always fire a fouling shot to warm up a cold gun, just before shooting the group.

This points up the benefit of a powder which is relatively temperature insensitive. Carefully controlling the temperature of rounds, to be fairly consistent, is challenging, and it's likely impossible to get it perfect unless you have time to pace your shooting. Some folks clean between every group, and precede every test group with a fouling shot, which also warms up the chamber. Then, by pacing the time between record shots, to prevent the gun from heating up too much, you can get a handle on it.
-

 

[jlow]

I have not yet confirmed this but I suspect it's due to the first round being chambered for longer and heating up. I usually chamber the first one before I sit down and get comfortable. Second one I'm already on target so time between cycling the bolt and firing is much shorter. Doesn't explain why the data I had for factory is affected less though.
I have a second chronograph that I'll be trying out soon. just to make sure.

That would do it.....

 

[Steve Ladino]

First shot velocity discrepancies are a feature of your barrel and how it handles cold bore (clean or dirty) shots. I generally foul/warm my barrel before testing individual loads. One rifle I have consistently shoots first shot slower.

Regards JCS

My experience is slower first shot, usually high.