I bought a used Weatherby MkV 340 a couple years ago. I did a cursory cleaning and worked up several loads for it with a 250gr Sierra Game King BT bullet. The accuracy wasn't real good with max loads for a couple of the slower burners (IMR4831 & H4831SC) but I found 77gr of IMR4350 to be pretty good with about 1 1/2" groups. I borrowed a friends Chronograph when testing these loads and they were all as good or faster than the load data advertised, including the 4350 loads. After last hunting season I decided to get serious about cleaning the bore and bought some Tetra Gun copper solvent and went to cleaning. There was a lot of visible copper fouling. It took a lot of effort and several overnight soakings to get the bore clean.
My wife just bought me a chronograph for my birthday and so the other day I went to the range with my clean gun, new chronograph, and some of the loads that I had built previously. I was pleased to find that it didn't matter which of the loads I tried, the accuracy was really good. Seems the bore cleaning helped. Then I chrony'd my old standby 4350 load and to my surprize found the velocity was down by 200fps. I then chrony'd a new load I had built and found that the velocity was off the advertised velocity by 130fps. I then tried a couple of factory rounds and velocities matched the manufacturers claims. I'm not sure now whether I got bad velocity data from my friends chronograph, my new one, or the bore cleaning caused a velocity change with the bullet I reload. Didn't seem to affect the factory loads, although I didn't have any of those in my original data.
Anybody have an idea?

edard,

Chronys are definitely a problem and they frequently read too high. Light seems to be the problem for them. Thus a particular Chrony will read correctly at location A but not at location B. However, if reading incorrectly at location B, they can still be used to compare velocities as in my experience the results are consistent for "that day" at "that location".

Mike

Howdy,

Mike is correct; however, I really think the culprit here is all the copper you took out of the barrel. Let me see if I can explain it. With all the copper in the barrel two things happen: 1. there is less room for the bullet, therefore pressure escalates, and 2. there is more friction down the barrel, therefore, again, pressure escalates. Now, you have to understand that more velocity is a result of more pressure. Therefore, with a baseline velocity measured while the barrel had two pressure escalators; it should be no surprise that when the barrel is clean, there are no pressure escalators, and the velocity drops!!!

I hope that is clearer to you than it sounds to me, but that is really what has happened to you. By the way, aren't you proud of the accuracy now!!!!?

Good luck and good shooting,

Coach

I think trading 200 FPS for a notable change in accuracy is a good thing. Keep that barrel clean.

Yes Coach, your explanation does make sense. I thought about this a little after I posted my message and it is reasonable to think that I changed the barrel characteristics for the bullet. I will have to do some more test loads to try to regain some of the velocity but I am going to hunt with my existing load because I have a lot more condidence going after game knowing the accuracy is really good. I'll be doing a regular cleaning after so many rounds from now on. Thanks for the input.

A couple years ago a friend of mine wanted to stop by before deer season to check his 30-06 bar.
I was shooting over the chrono and he had never seen one before and was real impressed at how it worked. He asked me to guess what I thought velocity would be with rem factory 165's. Before he shot I said 2725 fps. My mouth dropped open when the first one clocked 2900 and hit 6" high.
Then the next clocked about 2850 and hit 4" high.
The next clocked 2800 and hit 3" high.
After about 6 rounds he was down to about 2725 fps. And right by the bullseye.

I asked him how he cleaned and stored his gun after the last season. He said "I just slop a bunch of oil down the barrel and put it away".

NOW THE BIG QUESTION--did the oil make the barrel less slippery resulting in more velocity OR did it make more pressure as the bullet acted like a "high speed squeegy" roaring down the barrel. I think it made more pressure.

By the way--this guy hadn't sighted in his gun for 3 years and each one of those years he got a monster buck with the first shot of the year.
SOME PEOPLE HAVE ALL THE LUCK--and us guys that do all the prep work get.............?

Here's a couple other ideas. As a barrel warms up, velocity can drop somewhat.

Expansion.

What temperature did you do your shooting sessions?

Hot weather means higher pressures and velocities.
Always work up a load in hot weather.

One thing I have found in competitive shooting is that we do not clean our barrels down to the metal all the time it changes point of impact and velocity. What I used to do is clean after the match season down to the metal. from then on I just run wet matches after each match. No more than three or four.This was back ten plus years.
After discovering Tetra Lube and the cleaner is this. Clean bore with cleaner until patches come out clean. Next wipe with Tetra Lube. This will remove residue. when working up loads I take ten of the standarad loads that I know work. 1moa or less. I fire 3 and wipe with tetra lube fire 3 and wipe the 4 and wipe. You will notice the groups shrink. Next work up your loads as normal.When done with the days shooting wipe with 2-3 patches of the solvent the dri patches the 3-4 patches of lube. This works on new or old barrels.
By know you think it is an add yes it is but remember we try to stay consistant in all our loading and shooting practices why not our cleaning?