Cold bore conundrum!

[jcerne]

Hi All,

I trying out a new bullet in my Savage target rifle and am getting the best groups from a cold bore, which I’ve never seen before in any rifle. Usually, the cold bore shot is off a little and then the rifle settles down, but with Bart Avenger 68gr boat-tail bullets, the opposite is happening. The rifle is a Savage Target action with a 26" Shilen stainless select match bull barrel (0 freebore, 14” twist, ratchet groove) in 6BR Norma. I’m using a load that worked well for Bart’s 68 gr Ultra FB and Berger 68gr FB Target. The load was: Lapua cases (annealed), VV N133 29.1gr, CCI BR-4 primers, with bullet seated at jam or 0.005” back from jam.

All the shooting was done at 300 yards with very light wind and a temperature of around 34 F. All the groups were five shots. On the first day (please see Target 1), the first group from a clean, cold bore was around 0.6” ctc, which made me happy. Then the groups started to open up. I didn’t make much of this behavior until it happened again the next time I shot this rifle a few days later under similar conditions. The first group from a cold, fouled bore was about 1.0” ctc (please see Target 2), and then the groups started opening up. I didn’t the clean the bore before the second session since I only had 20 shots after the last cleaning and was curious to see how the rifle shot these bullets with the bore already fouled. On the second day, the bullets were at jam for the first two groups and 0.005” ack from the jam for the last two groups.

I’m wondering if the barrel is expanding slightly when warm, making the bullet fit a bit looser and causing accuracy to drop? Or maybe, the powder/cases warm up a bit in the pre-heated chamber, causing a slightly higher MV, which brings the rifle out of tune? I doubt this is the case since I’ve never seen that much sensitivity to MV in this rifle, but I’m running out of ideas. The only thing I can think of is to try some other powder weights at 300 yards.

If you have seen this before and/or have any ideas what is going on and how to fix it, please let me know.

Thanks,

John

 

[jcerne]

The last time I shot the rifle was with Berger 68gr FBs at a 300yd match in September, it shot a 99-2x (see below, 11 shots well within the 1.5" 10 ring), a 97-1x and a 99-1x, so I don't think it's any basic problems with the rifle or scope. Nothing changed except for the bullet, which looks really promising for the first 5 shots!

 

[Greg Langelius *]

Your load is optimized for a cold barrel. This is good for loads used in hunting where the barrel seldom gets hotter than lukewarm, when only a couple or three rounds are used in engaging your game.

Redevelop it for the warm barrel, and don't worry as much about the how's and why's. I'd be guessing a small charge reduction is called for.

Changing any component of a handload changes the load. Period. Redevelopment is therefore called for.

Greg

 

[308Gunner]

I agree with Greg 100%, I chased my tail for a lot longer than I want to admit.

 

[jcerne]

Thanks for the helpful advice. I never thought that one could optimize a load for a cold barrel, but seeing is believing!

 

[Old Soldier]

Check the torque on all of your screws.

 

[Chris_M]

I have seen large POI shifts while doing ladder tests at 600 yards with varying barrel temps. Keeping the barrel cool resulted in the ladder test producing a much lower POI on target. Shooting rather quickly and keeping the temp in the barrel a little on the warm side resulted in impacts of the entire ladder a few inches higher compared to the shoot/cool method. Same bullet and charge weights were used for both ladders and they were shot on the same day in the same conditions. This also showed different charge weights coming together in a hot barrel vs a cool barrel.

Cold on the left, warm on the right. definitely shows barrel temps change tune and POI, at least in this one small sample, on this day, under similar conditions. Also, I forgot to mention that there were no changes in velocity between the two ladders.

 

[Greg Langelius *]

To optimize for a cold barrel takes a chunk of lifetime, with all the waiting between shots. Carlos Hathcock used to go to the range each day and fire only one shot into a given target. That's dedication. It's also massive overkill.

This is one instance where a three-shot group is overkill.

The barrel harmonics consists of the changing barrel heat altering the amplitude of the muzzle swings. But those muzzle oscillations are only rarely in the vertical plane. They can be vibrating along a line that can be anywhere from 1 to 179, and 181 to 359 degrees off vertical. The harmonics part refers to the fact that secondary and tertiary (ad nauseam) harmonic swings can be occurring simultaneously, and can each be on unique planes.

Aside from that, as trajectory proceeds, the line along which these oscillations occur bends toward vertical to follow gravity. It's about physics, generally the Newtonian kind.

Now this may sound impossible to understand, but it gets a lot simpler if you break it all up into individual components. There are a lot of them, though.

I worked for a NASA optical contractor (Razdow Laboratories) back at the end of the 1960's, who provided the big telescopes for NASA's Solar Optical Tracking Network, as well as the Apollo Command Module windows. Those windows were my own personal responsibility. That was some chill stuff.

H-Alpha tracking

It was an exciting time to be a rocket scientists, but I was not one of those. I had been there, done that, back in HS, at Essex Catholic HS Science Club, where I was the Club's president during my Senior year (1963/64). I would have gone on to college, but I ended up getting drafted, into USMC.

I worked in fabrication. Some of those windows were spec'd to be flat across there entire surface to where dimensions that were measured to 1/4 the thickness of a wave of yellow light. It was a game of microns, optimized for minimum reflection, maximum transmission, and near-impossibly hard dimensional precision. But all the Apollo's flew with our windows aboard. That's why those all of those photos that got published had zero distortion, and also why Apollo 13's in-course corrections worked.

That's what I did when I was fresh out of 'Nam and the USMC, and 22-23 y/o. Now I'm 77 and 7 months.

But what was true then is still true now; break it down into the small parts, and complex concepts become very doable. Perseverance is the key.

Greg