I recently got two 6 TCU barrels; a 19" VVCG and a 21" Fox Ridge. The VVCG quality of work is excellent, the throat is centered like it should be. The Fox Ridge has a typical T/C off-center throat.

I loaded up some new .223 cases with 6mm bullets and went to the range. About half of the loads shot acceptable groups for fireforming loads; some of them less than 1" (this was the case for both barrels).

I went home and loaded up the now-formed cases, expecting them to shoot even better. I made sure to set the Redding sizer die for each barrel so the case head was flush with the breech, yielding about .002 headspace. This was when I encountered the first problem.

In both barrels, either the chamber was tight or the die was loose. After bottoming out the case in the sizer, the case was still not sufficiently sized to chamber all the way. I called VVCG about this. VVCG said they usually run chambers tight. No problem; I tend to like that. I called Redding. They said it happens some time and it's not a big deal. Just lap the die or shell holder until the case will size enough the chamber properly. I lapped the shell holder (since I have others) and marked it for this set of dies. I then proceeded to size the cases, making sure each lot was properly sized for it's barrel.

To my surprise, they shot worse (in both barrels). I kept the brass separate; each barrel has it's own lot. But it did not matter.

In the case of the T/C with an off-center throat, I'm inclined to believe the fire-formed case just exagerates the problem. A tight-fitting case just does a better job at holding the bullet in the wrong place.

However, the throat looks good in the VVCG barrel. But the groups opened up from 1/2 to 1".

Any ideas why?

liljohn,
If you used the same load as when you fireformed your cases you need to work up the load for the new case. It's volume has increased and so the powder charge needs to be increased. just treat it like a new batch of brass and work your load up for accuracy.

PaulS

Good advice, but I did that--increased charges from 1/2 to 1 grain. Didn't make a positive difference.

I also tried a different powder this past weekend. It didn't make a difference, either.

But a bad day at the range beats a good day at work, so I'll try something different this weekend.

I had this happen with my MKIV it seems that the sweet spot in acccuraacy is not all velocity, it may be initial bullet acceleration or harmonics it is like a sine wave, that it will come and go as to very little deviation in variables. I have learned to chrono fire form loads and found that I can repeat the accuracy at or near that velocity.

[ 05-13-2003, 16:31: Message edited by: Pokerplayer ]

The loads I put together were right around where I was with the fireforming loads. In the fireformed cases, I started with the fireforming load and went up slightly. The velocities are very close according to my chrono.

You might be on to something here with the mention of harmonics. I'm using a setup similar to VVCG's hanger bar, only I've used a block of hardwood and spacers as a forend. I like it because the barrels are fully floated and the hardwood block is flat on the bottom. Makes bench shooting easy. But the barrels actually ring like a tuning fork after firing.

But, I would not think this variable would be affected by a slight difference in case dimensions. I tried various ways of dampening the vibrations. In one effort, I shimmed the barrel and forend temporarily with a piece of leather-wrapped brass. It stopped the vibrations, moved my impact point up 12", but did not tighten the group.

One thing that does appear to occur--some groups seemed to tighten up as I increased my grip on the rifle. If I hold very firmly, and hold the rifle in the sandbags, some groups improved.

I wonder if it would be worth replacing the makeshift (but effective on the bench because it's flat) forend with the actual hanger bar components from VVCG? And perhaps filling the hollow rynite stock with lead shot? Maybe the variation is coming from movements in the wood and barrel vibrations? Does anyone think added rigidity and weight would dampen the shifting?

I've already got the Bellm hinge pin so the barrels are tight on the frame.

Size your cases in lots of five while adjusting the sizing die a few thousandths per lot. This will find the sweet spot as to how your barrel likes the cases sized. My 6.5T/CU did this and by experimenting with the sizing it shoots formed cases great.

REF:

"Just lap the die or shell holder until the case will size enough the chamber properly. I lapped the shell holder (since I have others) and marked it for this set of dies. I then proceeded to size the cases, making sure each lot was properly sized for it's barrel."

If it is a length problem, remove material from the end of the die in order to shorten it and thereby move the shoulder of the case back.

If you remove much material from the top of the shellholder, you will weaken it and make it more prone to stripping out. Best to not thin the shellholders.

Die manufacturers often make the dies too long inside so you cannot push shoulders back and create an excess headspace situation, but in the process, you cannot then bump shoulders back enough for these break open guns.

"Lap" hardly describes the process. Either face off the bottom of the die in a lathe with carbide or grind some off with a bench grinder. Make sure there is some bevel remaining at the mouth of the die, and if there isn't polish the edge rounded and smooth. Then be sure to douche out all the grit inside the die before using it.

Get the headspace as close to zero as you can, but short of the breechface shoving the case into the chamber when closing the barrel.

Mike