View Full Version : Chronograph problems

Rich K
12-31-2007, 06:44 AM
Wasnt too sure which forum section to post in so opted for here?
I have a F1 chrony and have had a few bizarre outings with it and was wondering if anybody has had anything similar or can shed any light on it?
I was testing my 300 win mag with a short barrel and moderator. The first 2 rounds were 2885 then 2887, thought lovely thats looking good. Next 2, 2930 and 2929. I wasnt too happy and left the rifle for 10 minutes wondering wether it could be anything to do with heating up. The 2nd string went: 2880, 2887, 2946, good time pause for 2887, then 2913.

Should one allow a time period between shots and keep it consistant??

Cheers Rich.K

12-31-2007, 11:54 AM
When you say moderator you mean silencer don't you.Maybe it's expanding from the heat of the first two shots and not sealing well till it cools down some.I know when i was in the service in the 60's, I had a silencer on a XM21 and after the first couple of shots, it would get louder and bullet impact would change some,and the can would get hotter than the barrel from the trapped gases.
Rudy Manuel

Rich K
12-31-2007, 01:22 PM
Not sure if it is the mod? I know I was shooting at a reasonable pace for accuracy and the rifle shoots 0.5 moa all day long, the moderators/silencers do get hot. My friend had one on a ultra mag and after 6 shots you wouldnt want to touch it!
I think the first 2 rounds are accurate velocities, it matches with info on powder charges from manuals. I will go with those for the moment. Will try and borrow another chrony to check against

Louis Boyd
01-01-2008, 03:50 AM
How far is the first screen of your chronograph from the muzzle? If the propellant gasses reach one or both screens before the bullet it will give eratic readings. Though the shockwave from the propellant isn't very dense optically it has a lot larger area than the bullet when it reaches the screens. For a 300 WinMag I'd want at least two and maybe three meters to the first screen.

Rich K
01-01-2008, 04:13 AM
Loius depending on site I try and set up about 12 feet away. I think on this occasssion I was probably about 3 metres, 9 feet approx.
Its not something with the chrono is it, if it measures it should be right as long as set up is correct?

01-01-2008, 04:57 AM

Look like perfectly normal extreme spreads to me :confused: You did say short barrel..........shucks, thousands of competitions have been won with ES's like that. Folks around here talk about Extreme Spreads "in the teens or single digits" as if it's normal, it ain't.

I can't see the problem, why assume it's the moderator OR the chronograph?


Rich K
01-01-2008, 06:30 AM
thanx for the reply, yeah I know what you mean but it just seems the first 2 shots are really good then seems more random after that. What are the chances of the first two only being good? Statisicaly I need to shoot a much bigger batch and also probably one with the moderator off.
My friend shot a short string with his 6.5 x 55 which has no mod, the results seemed much more 'random'

01-01-2008, 06:41 AM
I agree with alinwa, thats a 61 fps spread a while thats for me would be on the outer edge of extreme I have seen for worse on my own chrony, it will take some work to get it worked down into even under 20 fps, but for a 300 win mag I dont think its much to worry about, accuracy will still be good if the load is right, I had an extreme spread of 120 fps on one rifle but it was still punching 5 shots into .3 of an inch and 100 yards and still 1 1/4" at 300 yards (though I did fix it with a ifferent powder and got it down t under 10 fps, just couldnt live with it) the extreme spread may show after this but up to 300-400 yards it wont show up much. I am not really sure wha causes bad extremem spread but I hav a feeling its a couple of things

Firstly just plain old powder, the rifle that had 120 fps spread was using H322, I changed to BL-C(2) and extreme spread improved dramatically), we know
H322 is a great powder but in this particular application it just didnt work.

Neck tension plays a big part, this is where bushing dies are handy, I experimented with different bushing and to a certain point the less tension the less extreme spread, though if your rifle is mag fed you have to be careful if you go too light youre bullets could be pushed back when cycling from the mag, I am running with about 0.0015 neck tension, but if youre feeding from a mag no less than 0.002 should be safe.

Whatever you do dont burn up that barrel chasing low extreme spread figures, hae some fun with the awsome power of the 300 win mag, I would love one myself but am slightly afarid of the big magnums, when I need a slap in the shoulder I go shoot the .303 and then I remember I dont like recoil!

What are you using it for Rich, f-class or hunting?

01-01-2008, 06:47 AM
Chrony's need a first "shot" through over the screens to calibrate. You can either shoot a rubber band over the screens or sacrifice a shot to get it to calibrate. Yours may be ned ing two trips over the screens to calibrate. Try the rubber band "trick" and see if it helps.

This drove me nuts for a while until someone put me on to this. I kept thinking the first shots were about 100 fps low and was caused by the clean barrel even though they shot well at distance on paper.

Hope this helps.

01-01-2008, 06:48 AM
Al's right....Al's right???? Yes, he certainly is. For a large capacity cartridge and a short barrel I'm surprised the ES is not greater than the 60 fps or so ES you report.

Part of the problem here is the large powder charge and the short barrel. With that chambering you are still burning powder at 24". I am assuming the short barrel you have is in the 24" range.

Rich K
01-01-2008, 06:49 AM
Am definitely not too worried about it, its just really for working out theoretical drop tables. Bad/ good ES vel it still shoots very well. 1/2" at 100 yards is a ok group, most under. I sue it Criag for hunting and paper punching. I had the barrel cut down to 20" as the mod adds 4", had made it so much better balanced. A5 stock, leup scope, all in weighs 14lb

Rich K
01-01-2008, 06:55 AM
Travelor, cheers for that, wasnt aware of that issue at all.

Jerry, if 24" is short, this ones a tiddler!! :) It was 262, now only 20". The moderator was unbalancing the rifle and made it a pain to hump about. I will put ES to the back of my mind as the 178 A max are extremely accurate on paper and drop deer perfectly. I rarely shoot past 500 yards so the ES wont really effect me.
For a large case, 300 win mag, 300 ultra, 378 etc.... what is a good ES??

01-01-2008, 10:15 AM

Low ES, like a one caliber hole for a 5-shot group, is just something to strive for. You are not probably going to get there but you are better for trying.