I recently took delivery of a custom .300 WM. It's built on a Wby MK V action with a 24" No. 4 Hart tube (.330" chamber) in a hand-laid McMillan stock. I added a Jard 11 oz trigger and a Tubb Speed-Lock kit. Inital test groups from my 'smith hovered around 3/8"- pretty good. Not being one to let well-enough alone, I went after the barrel channel with some 220 grit paper as there were numerous (and irregular) contact points along the length of the tube. I also glass bedded the channel under the chamber-portion of the barrel. Groups went to hell. I then added a glass pressure pad at the forend and now groups with proper loads are one ragged hole. I am throughly happy with its performance but am wondering if this is a common configuration for glass bedding. Also, I notice excessive pressure signs (0.004"-0.005" head expansion) well before I reach published maximum loads. Could this be due to the tight chamber? TIA.

I think that both of the questions you ask are questions for your particular smith. He would know whether he chambered tight or not, and could also tell you why he bedded the rifle as he did.

quote:

I notice excessive pressure signs (0.004"-0.005" head expansion) well before I reach published maximum loads. Could this be due to the tight chamber?

Many barrels seem to shoot well with slight pressure points on the barrel.....

While I much prefer to float the barrel I know that some shoot better with the pressure appropriately applied.

I doubt seriously that the expansion you refer to is caused by tight chambering...there's something wrong...have it checked out.

quote:

Originally posted by Juggernaut76:
Also, I notice excessive pressure signs (0.004"-0.005" head expansion) well before I reach published maximum loads. Could this be due to the tight chamber? TIA.

Expansion measurments between unfired and fired MIGHT tell you somthing about the radial clearance in the chamber, but it sure as hell isn't a pressure sign. Fire a factory load (or 3) and measure the expansion ring, to .0001 (1/10 of a thousanth). Now see how that number compaires to your reloads.

quote:

Originally posted by Juggernaut76:
I notice excessive pressure signs (0.004"-0.005" head expansion) well before I reach published maximum loads.

Seeing pressure signs before reaching published maximum loads reminds us that the published data is to be used only as a guide. I have hit maximum pressures before and after reaching published maximums. And the reasons are many.

It could be that the chamber does not have the freebore that factory Weatherby's have. I had a Mark V rebarreled a few years ago and got significant pressure signs with Weatherby factory ammo. Sent it back to the 'smith to lengthen the throat. That solved my problem.

I vote for more freebore(longer throat) to fix the problem. Of course that might raise hell with accuracy...You might try seating the bullets .02" deeper, if you are seating them out to touch the rifling-it's almost like a longer throat, but cheaper.

The throat was cut to position a 190 gr Sierra MK about 0.005" off the lands when seated to the base of the shoulder. Therefore it is quite generous as the projectile is fairly long.
RL22 seems to produce the most obvious pressure signs when loaded near maximum. Powders like 7828 and H1000 seem to midigate the effect. Velocities with all powders are considerably higher than those listed in the manual for a particular charge. Yesterday, I chronographed a load of 75 gr of RL22 behind a 165 gr Nosler Ballistic tip for an average velocity of 3232 (listed velocity is 3110) and an SD of 6.0. Accuracy ran in th .2's for 5 shots fired at 5 minute intervals. The fired cases extracted easily and exhibited no head expansion. All things considered I think I'll leave the chamber alone (as accuracy is outstanding) and work loads up in very small increments. Thank you all for you input-it's nice to have such an extensive knowledge base to call upon when needed.

Jugg, what do your loaded ctgs. measure at the neck, if not thou.or so, that could be your problem.

I turn cases to .010" neck thickness, resize to .327" (with Redding Competition bushing neck die) and loaded ammo measures .328" @ the neck. Chamber is suppose to be .330"- though I have not made a chamber cast.