Bad Chronograph

[Bigcat_hunter]

I shoot a 338 ultramag with 300g Sierra Matchkings. My chrono says 2770 f.p.s. consistently. I follow my drop chart and it hits low about 5" at 500 yards, 6" low at 600, 7" low at 700 etc. It seems to shoot as if the velocity were more at 2650 f.p.s. or so. Could the chronograph be faulty? I am using a "shooting Chrony" brand chronograph.

 

[dirtball]

Best way to find out is to find someone elses chrono and put a few rounds thru it. Preferably with yours right behind his and shoot thru both at the same time.
Dave

 

[tjonh2001]

have you thought that the chrony might be right and that the BC on your bullet isnt what you thought it would be. i know that i use berger 168 and they say that they are .648 and i have done alot of testing on this gun bullet combo and its more like .64. this might be your problem if its not the chroney

 

[specweldtom]

Big cat, what is your barrel length? I shoot 2760 fps (with the start screen at 21 ft) with 300 gr smkhp's in my .338 x .378 with a 27 1/4" barrel, so that sounds in the ballpark. If you can shoot through an Oehler 35/35P, try that, but if the actual drop is greater than it should be, then the downrange velocity is for sure lower than you're using in the drop table. Could be muzzle velocity (chronograph reading too high) or lower B/C or both.

If you have a rifle that shoots on the drop table, try it through the chrony and see if it reads higher average velocity than the drop table velocity.

Good luck, Tom

 

[Bigcat_hunter]

Big cat, what is your barrel length? I shoot 2760 fps (with the start screen at 21 ft) with 300 gr smkhp's in my .338 x .378 with a 27 1/4" barrel, so that sounds in the ballpark. If you can shoot through an Oehler 35/35P, try that, but if the actual drop is greater than it should be, then the downrange velocity is for sure lower than you're using in the drop table. Could be muzzle velocity (chronograph reading too high) or lower B/C or both.

If you have a rifle that shoots on the drop table, try it through the chrony and see if it reads higher average velocity than the drop table velocity.

Good luck, Tom

My barrel is 27" with the muzzle brake. I set my chrono at 10 feet from the muzzle. I am just using Sierras BC info so I don't know.

 

[davewilson]

i'm not gonna say your rum can't shoot 2770 with a 26" barrel, but i'd bet it's pushing brass real hard to do it. 2650 would seem like a much more realistic speed to me.

 

[royinidaho]

My RUM shoots the 300 SMK fairly close to the BC at least to about 900 yards. However, I haven't tested it enough to for the BC to be validated past the first decimal, as in 0.X00

With 101 gr of US869 she goes in the 2730s over my shooting chrony which has been trustworthy except for skipped shots.

 

[Bigcat_hunter]

My RUM shoots the 300 SMK fairly close to the BC at least to about 900 yards. However, I haven't tested it enough to for the BC to be validated past the first decimal, as in 0.X00

With 101 gr of US869 she goes in the 2730s over my shooting chrony which has been trustworthy except for skipped shots.

What powder are you shooting the 300 with at 101 grains? 92 grains of H100 is all my gun can shoot with out showing pressure signs.

 

[specweldtom]

Bigcat, Roy is shooting US 869. I have a 115 gr load with that powder and the 300 gr Smkhp that chronographs 2780 avg velocity. The load I'm zeroed with is 110 gr of H-5010 and it is the one that averages 2760 fps. It also has single digit standard deviation. With your brake, the effective length of your barrel is probably only 25 1/2" or so. It is not likely that you can get 2770. I think you would have to push it to get 2650. I have 27 1/4" of effective barrel plus a home made brake for an overall barrel length of just under 29".

I routinely set my first screen at 7 yds to minimize muzzle blast and shockwave error. It makes the shooting window smaller, but doesn't rock the skyscreens as violently.

You're not using a B/C of .8 are you? Sierra downgraded the B/C awhile back to .7 something.

If you shoot it across another chronograph, let us know the results.

Tom

 

[Bigcat_hunter]

Well I cant find anyone locally that has a chrono I can use. I went and shot through my chrono today and in a five shot group it read as so;

2738
2748
2758
2745
2755

So I don't know. I guess all I can do is print out a chart using a lower velocity and hit the range and see if its on. I know where my bullets are hitting at least and the velocity would be around 2650 or so. What a pain. I am going bear hunting in a couple of months and need to get this dialed in for practice.

 

[royinidaho]

Bigcat_hunter,

You seem to have a very reasonable load.

Average Velocity 2749
Extreme Spread 20 FPS or +/- 10 FPS from average velocity
Standard Deviation 8

I'd keep doing what your doing and change BC and/or velocity to make things match up.

Another thing you could try is sending some 22 long rifles through the chrony.

I seem to remember, not sure by a long shot, but something like 1050 or so from some of the 22 rimfires. 1048 keeps coming to mind, but there are questions about my mind, especially the memory part.

Maybe someone will chime in with a decent number.