700ML headspacing

[fishhawk]

Following is a copy of an e-mail that I sent to McGowen after their head gunsmith John requested a recoil shoulder to breechplug primer pocket tip measurement. With the few actions and bolts that he had seen that all had varying specs he was not perfectly confident in choosing the installed depth of the breechplug for the DMB barrels. These specs I gave him are in line with the Pacnor 700ML DD barrels and will work fine with the headspacing directions. I have posted a pictured tutorial of how to headspace DMB and DD barrels below (reply #3). Thanks! Dave D for the lesson!

Dan, please see that your gunsmith John gets this info. John, after measuring 2
Pacnor 700ML barrels with 5 different Savage hex-head breechplugs, I found that
the measurement from recoil lug shoulder of the barrel to tip of primer pocket
ranged from 1.824"-1.830". This spec range is also what I want for the DMB
barrels which are also intended to use a .186 lug. At this spec the bolt likely
will not close even without a primer. After speaking with the designer of the
Pacnor barrel I was enlightened about how he intended to headspace. The bevel
cut in the nose of the bolt is approx. 43 degrees from centerline of the bolt,
the bevel on the primer pocket of the breechplug is 50 degrees from the
centerline of the plug. The angle on the breechplug needs to be re cut to 43
degrees or slightly less and continue cutting at that angle a little at a time
until the bolt will close (with the retaining screw in place)even if the overall
length of the breechplug is reduced slightly. Then a new slight internal chamfer
added to help start the primer into the hole (like the plug had originally).
With this setup the bolt is not likely to close with a primer installed. Most
209 primers can only be pushed by hand about 1/3 to 1/2 their length into a
breechplug, the primer pocket is to be polished/lapped/sanded what ever to get
the primer to push in the plug 3/4 or more until it will allow the bolt to close
with primer installed. This setup allows much more of the primer to be
surrounded by metal to prevent leakage or pressure problems. I will be posting a
pictured tutorial on our forum so our members know how this is to be done. Thanks! Greg Hawk

[ratsnakeboogy]

I had this issue after fishhawk installed the new lug on my .45 with Hunter's bolt nose kit instlled and the accompanying new breechplug for BH209. The bolt would not close even with no primer installed. I am far from a gunsmith and did not want to mess up the new breechplug, so after looking at it for a long while, I figured out I could headspace off of the bolt retention screw. I hand filed the diameter of the BRS a little at a time until the bolt would close with a slight crush on the primer. This may be a silly way to do it, but I have extra BRS's, but not extra plugs so I went with it. It seems to work well. Just thought I would throw that out here.

[dave d.]

:)Greg anytime it was a pleasure talking to you.

[fishhawk]

This is how to headspace a 700ML using Hunters boltnose kit, a Savage 10ML-II hex-head breechplug, and either a McGowen DMB or Pacnor DD barrel.

This is a bolt with Hunters boltnose kit, a CCImag primer, and a Savage 10ML-II hex-head breechplug. Note the gap between the boltnose with the primer pushed firmly in the pocket, lots of primer body exposed. With the parts installed in the action, especially the bolt retaining screw, the bolt is likely to be stopped by the breechplug not allowing it to close.

This is the crazy arrangement I made for recutting the angle on the breechplugs primer pocket. Air grinder in a vice, on a drill press. The ventliner is removed, and a piece of 10-32 threaded rod, washer, and a jamb nut are used to chuck the breechplug.

Here I set the angle to roughly 40 degrees so the breechplug primer pocket can go into the 43 degree angle on the end of the boltnose. The original angle was 50 degrees. Take note now of the very slight chamfer on the inside edge of the primer pocket to help primer alignment when bringing the bolt forward.

Here is the new angle lightly cut into part of the original 50 degree angle. The pointer shows a fine blued line of material that has yet to be cut. After cutting a little off, reinstall the plug and bolt (remember the retaining screw) and test for bolt closure. Keep cutting a little at a time until the bolt will close without binding or leaving a shiny spot on the angles from contact during bolt closing.

This is the breechplug cut enough to clear the boltnose when closed. The grinder actually started making light contact with the tips of the hex. The primer will only push this far into the pocket with firm thumb pressure, the pockets bore needs to be sanded or polished a little at a time until the primer will push in 3/4-7/8 of the way. Use the primers you plan to use as different brands have slightly different diameters or tapers.

Here's how deep the primer should push into the pocket when pushed by the bolt. You shouldn't be able to push it in this far by hand or the primers will likely stick when fired, especially fired primer only. The best results are when the bolt closes with moderately more resistance than when no primer is in place.

This is how much tighter all the parts should be able to come together when finished. Notice how much more of the primer body is surrounded and supported. If needed re-cut the slight alignment chamfer noted before if it was lost or lessened by cutting the 40 degree clearance. Install everything, and the bolt should close nicely.

[dave d.]

:)very nice Greg that is perfect. Nice work...

[GMB54-120]

Very nice and very similar to the NULA breach plug's angle cut. The primer is nearly 100% enclosed by steel.

This is without any compression. When its compressed the gap is almost nill. Even the bolt has a slight countersink for the primer "rim".


Primer seats deeply in the plug.


IMO this is still one of the best designs to fully support a primer and limit blowby or primer failure. Primer selection is a bit more limited but mine will handle about 4 primers very well.

[Deleted]

looks good Greg, good details and simple instructions....Bill

[dave d.]

I think this needs to be a sticky. What do you guys think?

[Deleted]

certainly....

[Richard]

Very nice description Greg!...........Man, a little mini-lathe would be the cats arse for you
Richard

[jims]

I just read this, well done.