Muzzle velocity variation

English Mike · Posted: Thu Jun 16, 2011 2:25 pm Post subject: Re: Muzzle velocity variation

I'll bet a £ it's chronograph error of one form or another:

Weak or failing battery.
Not using sky screens.
Bullet path not centralised over the pickups.
Chrono too close to the muzzle - I like a good 10yds.

gelandangan · Posted: Thu Jun 16, 2011 3:13 pm Post subject: Re: Muzzle velocity variation

Well, a page full of WAGs with no response from dhc yet..
Maybe the problem is solved Smile

Pumpkinslinger · Posted: Thu Jun 16, 2011 6:05 pm Post subject: Re: Muzzle velocity variation

Gelan, these are SWAGs, not mere WAGs!

stovepipe · Posted: Fri Jun 17, 2011 2:11 pm Post subject: Re: Muzzle velocity variation

+1 Chambered on the recoil...300'/s would be a huge diff in kaboom. The case would be hammers and the primer'd be toast if not blown out if the load was anywhere near MAX.

Same powders from different lots often have velocity deviations....but 300'/s? Yeesh!

I don't have a chrony. But if I got a good recipe I like I'm aware average POI can change with like powders/different lots. So if it groups ok no worries a click here or there gets it back on center and I dont take for granted one lot will work as the last so it's check the primer etc as I would a brand new load. None of mine are anywhere near MAX either so I got some wiggle room for hot days, a faster lot etc...I fail to see any upside to trying to get the last few FPS. I pick the middle or lower charge weight loads when workin' stuff up if it prints ok. A hit's a hit and so long as the bullet performs when it gets there then it's going fast enough. I know one lad that loads to the MAX on darn near everything pistol and rifle. He likes it that way and reasons today's loads are lawyered up and dumbed down so ...but not meee!!! I shoot way more than him and why beat the tar outta my guns and body ro risk a KB.

dhc4ever · Posted: Sun Jun 19, 2011 3:22 pm Post subject: Re: Muzzle velocity variation

gelandangan wrote:

Well, a page full of WAGs with no response from dhc yet..
Maybe the problem is solved Smile

Sorry guys, been away to sunny Melbourne for a few days with no internet.
I havent been able to catch up with my mate for the same reasons.
The posts all have interesting views and I will be trying to pass these onto him today. Looks like the unlucky bugger will have to drag his sorry arse back to the range and redo his testing, with a fresh battery for the chrono.
Vince he does shoot at Ripley and I suspect it was their chrono he was using.
Thanks all for your comments and advice, keepem comming if you think of anything else.
I'll let you know how he goes when he tells me.

Cheers

dhc4ever · Posted: Mon Jun 20, 2011 4:09 pm Post subject: Re: Muzzle velocity variation

Well I did manage to get onto him today.
It seems he has been using a digital vernier to measure his case lengths.
He changed the battery and the difference was .020", so consequently most of his case lengths are .020" over maximum length.
Now I cant see how this would have an effect as he is seating his projectiles .040" off the lands and as such his cases are nowhere near contacting anything important in the chamber, or am I wrong?
Hes going to trim a batch of cases and reload and with a new battery on hand for the chrono (if its needed) hes going to see what he gets variation wise next saturday.

Bushmaster · Posted: Mon Jun 20, 2011 7:34 pm Post subject: Re: Muzzle velocity variation

In that we don't know his chamber specs and case specs it is still possible that the cases are too long and the case mouth is into the throat and contacting the rifling leeds. This could have [not so nice] ramifications causing high and low (read near normal) pressures.

Just guessing here.

But, I will say that I would never trust anything that is battery powered. I use a "dial vernier" calipers and my powder scale is a RCBS 5-0-5 and a RCBS electronic [plug into the wall outlet] Powder Pro.

Ominivision1 · Posted: Tue Jun 21, 2011 12:06 am Post subject: Re: Muzzle velocity variation

I agree with Bushy, anytime you change seating depth, you change the load for the cartridge. I have played around with different bullet seating depths and have seen 50-100fps difference. There has been tests done by Norma, and others in which seating the bullet further out will result in an increase in chamber pressure due to lack of bullet jump and meeting resistance sooner (rifling lands) then a bullet seated normally. It depends on the gun but sometimes playing around with different seating depths can result in better accuracy.

chambered221 · Posted: Tue Jun 21, 2011 8:31 am Post subject: Re: Muzzle velocity variation

OAL of the case and OAL of a loaded cartridge are two separate issues that could both lead to increased pressure or spikes in pressure !!!

The reason for trimming a case to length is to keep it from entering the throat of the chamber. If the case is to long it could become wedged inside the chamber and not allowed to properly expand. When this happens the bullet is not allowed to move as freely as is should, thus causing a rise or spike in pressure.

Reference picture here

Notice where the case mouth ends and the throat begins, it steps down to a smaller diameter.
Typically this area is longer than the trim length to allow for some wiggle room. There are tools available to measure the actual length of this area. I've got rifles that are .030+ over trim length so this may or may not be the issue.

Seating a bullet into the lands is a widely accepted practice but you do have to use caution. It will also raise the pressure because the bullet is meeting resistance against the lands. Never move a bullet into the lands without dropping back on the powder charge and working back up.

Ominivision1 · Posted: Tue Jun 21, 2011 10:51 am Post subject: Re: Muzzle velocity variation

My brother bought a brand new pre 64 win in 270 caliber and had problems from day one. Handloads were very hard to extract and one box of winchester factory rounds were no better and case necks were cracking. We did notice some galling at the neck of both handloads and the factory loads. Dad decided to try trimming the cases from .010-.040, .010 at a time under the recommended trim size back then and viola at .040, never had a problem again with extraction. We then sent the rifle back to winchester and they provided my brother with a new rifle that is in specs and has never had a problem since.

Elvis · Posted: Tue Jun 21, 2011 12:51 pm Post subject: Re: Muzzle velocity variation

I had a friend with a .270 mauser action rifle that could only use norma ammunition as the other factory stuff wouldnt chamber well. If you compare factory rounds you will see Norma projectiles LOOK smaller for thier weight than others.

stovepipe · Posted: Mon Jun 27, 2011 7:22 am Post subject: Re: Muzzle velocity variation

Books a good place to start for trim-to lentgh. But, there's so many diff chambers so I like too...

Size then trim a case to TTL.

Press a bullet into a dummy case, smoke the mouth, etc.

Slowly close the bolt on it (NO primer etc...).

Slowly open it up..see any witness marks on the case mouth/bullet?
Adjust your lentghs accordingly.

If yer going JOTL and yer cases are too long you'll get a multi-piece gun before a sticky bolt.

I also NEVER trust them battery powered anythings....beam scale and a normal caliper work just fine fer this old fart.

Ominivision1 · Posted: Mon Jun 27, 2011 9:16 am Post subject: Re: Muzzle velocity variation

I agree with Stovey, I still use the normal dial calipers and 505 scale. In fact I was over at a shooting buddies house this weekend and he was loading up 100rds of .357 mag and it took him over 2hrs with his new rcbs auto powder dispenser!! wtf

The problem is with that unit is when it starts getting within 2 grains of set load, the unit trickles the rest of the powder ever so slowly.

I told him to send it back as it would take him till next Friday to load up 1000rds. Smile

Bushmaster · Posted: Mon Jun 27, 2011 9:24 am Post subject: Re: Muzzle velocity variation

And here I sit with my 20 t0 25 (can't remember) year old RCBS Powder Pro electronic scale. Weighing EVERY powder charge. Two hours to load 100 rounds? Who's in a hurry. I got all day and beings every day is Saturday morning I have time...

I do have a RCBS 5-0-5 beam for backup. Very rarely use it except for an occasional calibration check of the Powder Pro. Maybe once or twice a year.

stovepipe · Posted: Mon Jun 27, 2011 9:37 am Post subject: Re: Muzzle velocity variation

I like using my dippers w/ my beam scale. They even work when the power goes out!

You of all peep's should be in a hurry, GRAMPS. Shocked

Mornin' bud! Two cups in, off to the races!