I bought a new Defiance Tenacity with a PVA 6mmbr barrel. I took it out the weekend with some milder loads and every firing resulted in extremely heavy bolt lift and extractor swipe. The brass was new virgin Lapua sized to give 0.002 neck tension (no body sizing) with 28-29 grains of varget under a 109 grain berger hybrid set 0.020" from jam. All of the shot brass has an expanded case mouth that is 0.005" larger than brass shot out of my other 6mmbr and is actually shorter than the virgin brass. The body dimensions have increased 0.003" compared to the virgin brass. Thoughts?
New barreled action, heavy bolt lift after firing
- Thread starter crowellsr
- Start date
Yes lugs are lubricated. If this is high pressure this is going to be the slowest 6br barrel going as it barely cracked 2700 fps :-(
The brass that I tried will not be reloaded as the extractor shaved the base so they will be tossed. The chamber was clean and dry.
Mystic Precision
CGN Ultra frequent flyer
- Location
- Summerland, BC
Check the bore diameter.. it might be the smaller size?
Jerry
Jerry
It's supposed to be a 0.237 bore. I'll have to try measuring it when I get home but calipers are generally not the best for OD measurements
Mystic Precision
CGN Ultra frequent flyer
- Location
- Summerland, BC
Best is to use pin gauges. With the cals I use alot, I get a set so that is it 0.001" under, nominal, 0.001" over. Just drop them in and if they stop, that is enough info for me. Tells me alot about bore wear as the barrel ages.
You are not using a particularly heavy load so the pressure rise is just a bit too suspicious.
Good luck
Jerry
You are not using a particularly heavy load so the pressure rise is just a bit too suspicious.
Good luck
Jerry
- Location
- Western Manitoba
I will be following this one to learn - I thought the bullet was supposed to be at the groove diameter and that a pin gauge would tell me the top of rifling (bore hole) diameter. Only way that I know to measure groove diameter is to "slug" the bore and measure the protruding "grooves" on the slug.
- Location
- Western Manitoba
From your picture - I do not think those primers are showing excessive pressure, but definite smears on the case heads - is there a burr or something on your bolt face's ejector hole?
The "smears" that I have experienced are more like round shiny spots - I believe the brass was flowing into the ejector hole upon firing, then got sliced off when the bolt was opened - yours look more like a smear that was "wiping" along that brass.
Mystic Precision
CGN Ultra frequent flyer
- Location
- Summerland, BC
6mm is a quirky bore... you are right in that the bore is the hole in the middle of the rifling and there are 2 common diameters... 0.236 and 237". For a time, the 236" was desired as custom bullets where a smidge smaller and this bore made them shoot really well. Look at the Lapua 6mm bullet... was/is a smidge narrower.
Most common bullets want a 237 bore... stick them in a 236 bore and pressure go through the roof. This was something we ran into years back when the 6BR was becoming popular in F class.
If this is a pressure issue, and everything else makes sense... I would gauge the bore. 1 thou is not easily measured with calipers.
Another option if you have a friendly gunsmith nearby, try and insert reamer bushings into the bore and see what fits.
Jerry
In addition, measure the diameter of the solid portion of the case head and compare it with an unfired case. If the solid head has expanded, the pressures are indeed serious. Primers falling out would also be a warning sign.
Perhaps if you want to use Varget, you are going to have to cut back on the load, at the expense of velocity.
Try a different powder?
Perhaps if you want to use Varget, you are going to have to cut back on the load, at the expense of velocity.
Try a different powder?
Looking at primers to determine pressure is not a reliable method.
Hard extraction and brass flowing into an ejector hole are signs of excessive pressure. Any expansion of diameter in the extractor groove of the brass or the solid head of the brass is excessive pressure.
You experienced excessive pressure. The why is yet to be determined.
Hard extraction and brass flowing into an ejector hole are signs of excessive pressure. Any expansion of diameter in the extractor groove of the brass or the solid head of the brass is excessive pressure.
You experienced excessive pressure. The why is yet to be determined.
It could be a very tight chamber. I had a savage years ago that I experienced pressure even on minimum loads. Took it to my gun smith and checked it with go, no go gauges. The chamber was so tight that the bolt wouldn’t close on the GO gauge without some serious force. Is your barrel shouldered or a nut?
I would try to get your hands on some go/no go gauges and check it.
I would try to get your hands on some go/no go gauges and check it.
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