Extractor failing to catch rim of brass and rifling marks on projectile

[netcat]

Hey all, came across two issues during the process of loading some ammo for my bolt action rifle. Rifle is built on a Mack Bros Evo Gen I Stainless with a Proof Research Carbon barrel. It is chambered in 6.5 creedmoor.



I have some once fired Lapua brass that I recently resized. I had my die set to bump the shoulder back ~.003"-.0035" as my previous experience with starline brass required at least .004" for me to reliably and without force chamber just the brass. I am realizing now that the Lapua brass probably would have been fine being bumped back less as all the brass was super smooth at the slightly reduced shoulder bump. In any case there are a number of cases that when I chamber just the brass the extractor fails to grab the rim of the case about ~50% of the time. I have not had this problem with starline, hornady or norma brass in the past. Using my SAC headspace comparator set, the brass currently is the same shoulder to head length as a new hornady ELD-M factory cartridge, which the extractor catches just fine. Some things I've tried in attempting to fix/diagnose it:

• Dissassembling the extractor, cleaning and reassembling (M16 style extractor)
• Placing a piece of scotch tape on the back of the cartridge seems to get the extractor working properly
• Seating a bullet also seems to fix it
• Problematic Lapua cases rim thickness is around ~.0515"-.0520", functional Lapua cases rim thickness is closer to ~.0495"-.0510", Hornady onced fired brass and new factory ammo ranges between ~.0450"-.0505"
• Lifting the bolt handle up and down without cycling back and forth, causes the extractor to snap in place eventually
• Lapua cases that I have been having trouble with the extractor are pretty chewed up on the edge of the rim, unsure if from the repeated cycling during troubleshooting or if the extractor lip is riding on the edge causing undue wear and tear.
• No unusual resistance when chambering

My two working theories for this right now are:

1. Tolerance stacking in that the cases are probably a bit undersized for the chamber, and the slighly thicker case rim causes it to not catch reliably.
2. Being slightly undersized in conjunction with the ejector pushing the case forward enough for the extractor to fail to grab some cases, I think this is maybe the more likely answer given that it works fine with a bullet seated in the case.

I really don't think this is much to be concerned about, especially with having a bullet seated fixing it but I'd rather get a sanity check on it.


While troubleshooting the above I had an opportunity to have a dummy round chambered and the inspected upon removal. I noticed 4 marks on the projectile that are 90 degrees from each other, I am confident that these are from the rifling and not various chamber burrs as there are plenty of those and I know what those marks look like. Poor quality picture attached lol. Things to note:

• Lapua case + 140gr Hornady BTHP
• Jam measured via Erik Cortina's method CBTO of ~2.2250"
• CBTO for my two loads using these cases: 2.1700"
• CBTO does not change before and after chambering
• No pressure signs when load development was completed at the same CBTO

I guess this again is probably not a problem but I am really struggling to understand how the rifling would engrave the round this far back from "jam".

 

[smoooth1]

If your chamber is full of burrs, send it back!

Also, you’re not shooting empty brass cases; if they work once loaded, problem solved.