Clean / Cold barrel runs faster?

[corey006]

I am developing a load for a Ruger#1.(26 inch barrel) (YES, I know Ruger#1s are not a Long Range Rifle)

7mm Rem Mag.
Winchester Case(new)
CCI250
160 gr Accubond
Retumbo

My chamber has long throat. The lands measure 3.521.

My OAL is 3.506.(.015) off the lands.

I did a ladder test earlier this week and confirmed that around 70-72 gr was where I was getting tightest grouping.

This morning I went to the range and shot a few 3 shot groups.

When I got to 72.5 I was getting around 2940 fps. No pressure signs. This was after I had fired about 9 shots. I usually try to let the barrel cool a lot between groups. The barrel was probably still warm.

So I decided to clean my rifle. I dry patched and fired off 1 of my 71 gr fouling rounds.

I then shot my 72.8 gr 3-shot group. Much to my surprise the velocity was well over 3000.

I had 1 shot @ 3063, 1@ 3064, and 1 @ 3080. 1 of the cases had a slightly cratered primer.(Not bad just a little raised). So obviously that sign coupled with the speed shows I am probably a little over the max.

Both of these groups showed MOA potentional. (2 rounds @1/4 inch and 3rd hitting 1 inch away) the 3rd shots were probably be my fault.

So now the question.

When a barrel is clean and cold will it run 50-70 fps faster?...and can I assume that the 72.5 gr load may be actually closer to 3000 fps when it would be fired in a clean cold barrel. I am going to confirm this in next couple of days.


This is an early 70's Ruger so I suspect it isn't the best barrel. The rifle will shoot MOA when shooter does his part. 1 day I plan on re-barrelling it but it will have to wait. I shot a Mule deer buck at 328 yards last Fall(1 shot), and I can hit steel silhouettes out to 500 yards with it so I am happy with it as a 400 yard deer rifle.


Seems this rifle is very tempermental.

I think I will load a bunch of 5 shot rounds.(starting at 71.5 and slowly working up with 72.5 as a max.) I will make sure to leave the barrel cool more and clean and foul in between each string.

 

[Varmint Hunter]

I shoot all of my loads over an Oehler 35P chronograph. Without exception, every gun I own will shoot slower with a clean barrel. Presumably, no fouling results in lower friction which causes decreased pressure and less velocity.

On average, it takes about 3 fouling rounds (sometimes more) to get velocity up and groups stable and predictable.

I also noticed that the chronograph can give different velocities, on different days, with identical loads, fired from the same rifle. I believe that this is caused by varying light conditions on different days but I'm not sure. I have seen loads vary by close to 100 ft/sec over several weeks of testing. For whatever it's worth, I did send the chronograph back to Oehler for testing and/or calibration but they said everything was right up to specs when they received it.

 

[corey006]

O.K

So I am guessing it is a HOT or Warm barrel that will shoot slower?

Something doesn't add up because . 3 of a grain should not increase 100fps? If it does than that is one helluva pressure spike. Especially when the 6 previous groups showed consistent 20-30 fps gain every .5 gr increment.

I throw and individually weigh each charge. I am 100% sure there was no error on my part.

I will see what 72.5 gr generates for speed on a cold barrel.

 

[Pdvdh]

I shoot all of my loads over an Oehler 35P chronograph. Without exception, every gun I own will shoot slower with a clean barrel. Presumably, no fouling results in lower friction which causes decreased pressure and less velocity.

On average, it takes about 3 fouling rounds (sometimes more) to get velocity up and groups stable and predictable.

I also noticed that the chronograph can give different velocities, on different days, with identical loads, fired from the same rifle. I believe that this is caused by varying light conditions on different days but I'm not sure. I have seen loads vary by close to 100 ft/sec over several weeks of testing. For whatever it's worth, I did send the chronograph back to Oehler for testing and/or calibration but they said everything was right up to specs when they received it.

Roger that. That Oehler 35 if probably giving you very good data - it's top of the line in chronographs.

When I clean my bores, not with a dry patch, I mean clean back down to the bare steel, I have at times noted substantially lower velocity on the first clean bore shot. It was an eye opener to me, and helped explain why one should always enter the field to engage an animal at long range with a fouled bore. I figured it out when I took two different rifles to the range that I had already collected MVs for. When I fired the first shot out of the first rifle and saw how low the muzzle velocity was, I thought I had made an error and undercharged the case with powder. The next shot jumped up about 100 fps and fit right into the normal velocity range.

Low and behold the very same thing happened when I fired the first shot out of my second clean bore rifle. At that point the lightbulb came on!

Now - I will say that all barrels seem to be different and I don't see a 100 fps reduction in MV from all of my cleaned barrels. Some barrels aren't much different clean bore to fouled bore. But if I see any trend, it's that my clean bore shots trend toward slower MVs than the fouled bores.

Reason? I have read that the carbon coating layed down in the fouled bore presents less friction to the bullet traveling down the bore than the bare steel itself. I wouldn't bet my life on this being the cause, but that's one explanation I have read most commonly in the past.

 

[RT2506]

"I shoot all of my loads over an Oehler 35P chronograph. Without exception, every gun I own will shoot slower with a clean barrel. Presumably, no fouling results in lower friction which causes decreased pressure and less velocity.

On average, it takes about 3 fouling rounds (sometimes more) to get velocity up and groups stable and predictable.

I also noticed that the chronograph can give different velocities, on different days, with identical loads, fired from the same rifle. I believe that this is caused by varying light conditions on different days but I'm not sure. I have seen loads vary by close to 100 ft/sec over several weeks of testing."

I agree 100%. I have had the very same results except I have a Bata Master Chronograph.

 

[corey006]

I didn't type the part right about cleaning...

I meant to say I finished cleaning with dry patch.

I did the usual Shooter's Choice solvent and brushed it and kept running until patches came clean.

I then ran a few dry patchs then shot a fouling shot.

 

[eshell]

I've also found the first shot from a clean bore to be slightly (25-40 fps) slower, but all subsequent shots run very close to the norm.

After cleaning, I remove all traces of solvent with a patch wet with 91% alcohol, followed by a couple dry patches.

If using a chrome-moly barrel, you may wish to leave a trace of oil (or appropriate solvent) to prevent rusting, but it will typically require several shots to fully stabilize.

 

[Mikecr]

Corey, I don't think the jump is related to the cleaning. I think you've reached a pressure that accelerates the burn of slow powder, further down to bore.

Velocity is down with a cold/clean barrel.