First Shot Faster Than Rest?

[257Roberts]

I am posting this here, as I am shooting my 6.5x284. Using my MagnetoSpeed chronograph.

My question is: Has anybody ever experienced the first shot of a group being noticeably faster than all of the following shots of the group?

While this gun and load shoots lights out at 100yds, I've been struggling with ES over 50. I decided to back my load down in increments today and check some numbers again, looking for some consistency. Have been shooting 54.5gr H1000 with 150gr SMK. Decided to try groups of 5 @ 54.2, 53.9 and 53.6 grains just to see what happens. I know this isn't a large sample size, but I am trying to "find my load" while keeping my round count down with this barrel burner.

The first shot in every group is noticeably faster. I don't believe I was "cooking" any rounds, spending the same amount of time to chamber and fire each shot. Leaving only a few minutes to cool between groups. I didn't clean the gun or anything between groups. Even with the MagnetoSpeed bayonet attached to the barrel, these are all shooting 0.5MOA or less at 100 yds.

54.2gr, first shot was 31fps faster than the average of the next 4.
Counting all shots, SD/ES is 15/39. Counting just the last 4 the SD/ES is 9/18.

53.9gr it happened again, 56fps on the first shot over the average of the next 4.
25/64 on the whole group, 5/12 on the last 4.

53.6gr was more of the same. 40fps faster on the first shot.
18/46 on the whole group, 6/13 on the last 4.

When I see single digit SD that gives me hope, I just cant explain the first shots.
Can't say I've ever noticed this before. Or maybe it just hasn't been this obvious.

Making me want a guaranteed sighter before every string!

Be curious to see what others have experienced.

 

[CA_Shooter]

Yep, I saw this same thing with a 6.5 creedmoor but it stopped...was probably random chance.

I was using Hornady brass, H4350, GM210M primers, 140 ELDM.

I haven't chronod that load in quite some time so I'm not sure if that phenomenon has stopped with that load. I think it was just random chance that it happened

since then I've changed components that were chronod and I don't get it anymore

First shot highest FPS?

Was about 60 degrees tonight and I was doing some group testing. Weird results - my first shot of each group 0had the highest FPS. Anyone have any ideas? Saw another thread that mentioned if you leave the round in the chamber longer it could cause a higher velocity. Thoughts? 40.7...

forum.accurateshooter.com

 

[Gil P.]

I just started experiencing this. I took some time off of shooting due to the component shortage. This happened to me on the first round from a cold bore, and the first round of every group I shot after taking a break and letting the barrel cool.

I'm thinking it may be due to shooter error. My initial position is wrong and after experiencing recoil for the first shot in a group, I'll readjust slightly and my velocity will change.

How you manage recoil could change velocity. I'd try to be very consistent with every shot and see if it happens again.

I also have a Magnetospeed.

 

[XTR]

I think you have ID’d the issue.
I haven’t used a magnetospeed in a while but a few yes ago I was doing velocity testing and not trying to shoot groups. Just kneeling by the rifle and tripping the trigger. I found that to get consistent readings I needed to put my knee behind the buttstock. I believe part of the error is because the MS detector is attached and recoils with the rifle so you are getting relative motion in the detectors depending on how the recoil is managed

 

[Orr89rocz]

Interesting thoughts on the recoil. I have many rounds over a magnetospeed with multiple calibers from 257 to 458 and have not experienced this phenomenon. When the load is good my velocity prints pretty consistent. I have not noticed a trend of outliers in My groupings
i suppose you could try adjusting the magneto position and strap tightness and play With how you manage the recoil.

 

[Jaa600]

My GUESS would be it has to do with bore size. When you shoot your first bullet the barrel is cool. Perhaps the barrel/bullet interface is tighter. As you shoot and the barrel heats up, the barrel will expand which loosens the fit between the barrel and the bullet. This will allow more gas to escape around the bullet instead of pushing the projectile. Curious when size barrel you are using. Purely a guess. I have never tested it.

 

[Average joe]

I have experienced this phenomenon on many occasions. I don’t have an answer.
Perhaps a cooling of the barrel between groups? Although, sometimes the time between groups was only 1-2 minutes. I’ve gotten to the point of “dumping” the first round off to the side, and then shooting a group.
Interested to hear what others say on the subject.

 

[Longrange57]

I have been shooting with a magnetospeed since they were available. I shoot multiple calibers and multiple powders. Never logged a clean, cold bore shot that was faster than other shots that I chrono'd. If I clean all the way to metal, often I will see 80-90 fps slower. Without the barrel fouled, you have more gasses escaping and not as much energy behind the bullet. After 1-3 shots, in my experience the velocity settles down.

When going to next string, If the barrel is warm, and not the first shot of the day, it remains pretty consistent.

I looked back at my spreadsheets with 150 smk in my 6.5 x 284 with H4831sc and what I described above is dead on. I just shot last week with both 6Dasher and 7 RSAUM and both were 60-70 slower. The SAUM uses H1000.

Others may have different results for one reason or another. These are my results across thousands of rounds. It may not fix your problem, but try a faster powder like H4831sc or equivalent and see problem persists. Otherwise it may point to scales or some other anomaly.

Others on here may have seen your experiences as well. Hope you get it fixed!

 

[tsmith]

I don't have an answer for you on your issue. However, touching on what some have said about inconsistency of speeds when shouldering the rifle or just squeezing the trigger, un-shouldered. I have not had any issues with that. But I have my V3 on the barrel tighter than Dick's hat band. I've also never had the issue of the first round out of the barrel being any faster than the others to where is would make me think hmmmmm
.

Now to the ES over 50 the OP mentioned. I would be a train wreck if I had a load with 50 ES. But I guess if it shoots good, lay heavy on it. The load I just ran over 6 shots yesterday for a .308 I'm working with was a S/D of 2.8 and ES of 7, spitting 175's at 2768. Won't know the true speed until I true the MV tomorrow though.

 

[SweaterDood]

If you have the ability. Confirm your drops for 600+ yards, let the barrel cool and shoot a group. If the first shot is higher than the rest, you are experiencing something real. If not, there is something influencing the chronograph. I should also mention to remove the chronograph when shooting a group. The magnetospeeds can slide on the barrel changing the tune between shots