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Where has my velocity gone??

 
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Old 07-28-2007, 01:45 PM
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Join Date: Sep 2005
Location: Shoshoni, WY
Posts: 143
Where has my velocity gone??

I don't know if this is the right forum for this or not, so feel free to move the thread. I am not new to rifles and shooting, but to be honest I have never kept a rifle long enough to turn a barrel into trash and I only started chonographing loads a few years ago. I have a couple of questions that are somewhat interesting to me, but they might be just business as normal to you folks.

First off, I just received a new barrel job from Pac-Nor in a 7mm Remington Magnum. The rifle is a Remington with a 26 inch tube, number 5 taper, 9" twist. I decided to try the Berger 168 VLD bullet for hunting. The chamber is on the short side so the bullet does stick down inside the case quite a bit. I worked up a load of 64 grains of Reloader 22 for a velocity of 2950 fps. The velocity is shy of what most manuals publish for various 168 grain bullets over that much powder, but I am getting way under one MOA and there is enough energy to serve my purposes so I am very pleased. I got similar velocity results with other powders as well. I am curious, do you suppose the velocity is a bit low for the pressures because the bullet takes up so much case capacity?

I then decided to try the load in my 7mm Sendero. I tried H-4831 SC, Retumbo, H1000 and Reloader 22. I got the best accuracy with Reloader 22 so I worked up to 64.7 grains seated .010 off the rifling. Before going any further, I can tell you I bought the rifle used and just from the feel of a patch going in the bore it's obvious the throat is well eroded. On paper I am getting sub MOA, but when I shot across the chronograph I came up with 2816 fps. Wow, plenty of thump in the shoulder, but nothing down range.

I ran the velocity and conditions through Exbal and came up with a drop of around 39.3 inches from 200 to 500 yards. I was at 200 and the target backer was about 42 inches high so I zeroed at 200 and shot a three shot group that measured about .6 center to center. The group was a tad high, but I accepted it as close enough because I just wanted a ball park confirmation that the chrono was working. Off to 500 yards. Three shots at 500 and I had a group with a vertical spread of around .75 inches and a horizontal spread of just under 2 inches. The group dropped 38 5/8 inches so I assume the chrono was OK. FWIW, I had some factory 150 grain Winchester Supreme Plus (the old moly stuff) in my range bag so I ran that across the chrono and it clocked at 3163 fps. That same ammo made 3223 in the Pac-Nor barrel.

As you can imagine I am scratching my head. I am assuming the throat is so eroded the gas is bleading off with the boat tail VLD. Is that a correct assumption? But I am wondering how the rifle can maintain it's accuracy with the throat so trashed out the velocity is in the dump? What gives? Time for a new barrel?
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Old 07-29-2007, 04:09 AM
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Join Date: May 2001
Posts: 920
That 60 fps loss is NOTHING.

Rifles do not hve the same velocity with the same ammo - even if they're brand new.

Some years back, I chrono'ed a group of 308 rifles with Fed GM match. The high was 2725 and the low was 2550. All rifles were low round count, good barrels.

27-3/8" = 2725 fps
26" = 2550 fps
26" = 2570 fps
24" = 2685 fps.

So don't worry.
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Old 07-29-2007, 08:14 AM
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Join Date: Apr 2007
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I have an FN Mauser that I had rebarreled to 264Win Magnum (parent case for the 7mmRM) not too long ago and haven't put very many rounds through, with a Shilen barrel. I loaded some 140 Hornady SP's with H4831SC and at 3000 ft/sec they started to group pretty decent and in my judgement were at the threshold of leaving safe operating pressure. I am not a master gunsmith but have had a couple of rifles that were shot out and though they showed reduction in velocity the most tell tale sign was the accuracy went south. These rifles would shoot a group that I would consider acceptable for normal big game hunting out to 350 yds, IMHO at least, and then the next group would open up further and the next group would open up more. Even while allowing the barrel to remain fairly cool after the third or fourth four shot group they would measure 4" to 5" at 100', and the first group from a cold barrel would measure 1" to 1 1/4". I think you should get some Eliminator cleaning solvent for your barrels and give them a good cleaning. I have seen Eliminator clean the throats on match rifles that were thought to be "shot out" to bring them back to their youthful accuracy. A barrel that has been shot many rounds will build up fouling in the throat that becomes like glass from the heat and ammonia based solvents won't remove it but Eliminator if left to soak and then remove and then repeat will get it down to bare steel. Eliminator has no ammonia in it and therefore will not "eat" the carbon out of steel when left for extended periods of time the way most commercial bore solvents will. I am not in any way involved with this product but have seen the results of its use on match rifles that had many, many rounds through them. The test of a load for me is consistant and repeatable accuracy and if the velocity is on the higher end, well, so much the better. Some barrels are just faster than others but groups is where rubber meets the road. Good luck and safe shooting
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Old 07-29-2007, 07:29 PM
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Join Date: Sep 2005
Location: Shoshoni, WY
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"That 60 fps loss is NOTHING."

I only mentioned the difference between the two rifles with factory ammo as proof that the chrono is working. My concern is why I can barely get 2800 fps out of the Sendero with 168 VLD's. FWIW, I chronoed some loads today that I tested about a year ago and the velocity loss is about 220 feet per second. I guess my question is at what point do I rebarrel?
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Old 08-09-2007, 04:45 PM
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4 me it is when the accuracey goes away...
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