neck clearance question and safety

[djtjr]

Guys i have a 300 WSM on McRees chassis that is sickly accurate. its just a freak for a long range hunting gun. when i got it i was told it was a no turn chamber and (perhaps foolishly i believed it) i loaded up some Norma brass with H 4350 and Berger 210s. No matter what combo i shot it shot 1/4 min all day long. i just seem to have gotten really lucky. the one issue i had was that of the 100 rounds 10% wouldn't chamber. I assumed perhaps too long etc but everything miked out fine. In the end those 10% were maybe .0005 thicker in the necks and that was preventing chambering. With a loaded round OD of .338 my fired rounds actually measured smaller at .337 or .3375. Thats clearly not a great place to be but it is so accurate the best node is basically bench rest quality 1 shot hole so I am reluctant to change too much. I will if it is not safe but i filed a bunch of rounds before noticing this and carefully watched for pressure signs. I don't know what the actual bbl chamber measures but the rounds that fed in did go in fine with out any marring so i am assuming .001-.002. I know the old saying about assuming but if i find it to only be .001 or less despite no pressure signs what are your thoughts? My initial logic was that if it fed with no resistance and extracted accordingly I was good but perhaps came here for a voice of reason. Its just hard to mess with a perfect shooter. I took .001 off the ones that wouldn't chamber and now they do i may take a bit more off .002 and see if she still shoots the same. Given there were no bad nodes i am hoping the luck stays and she just happens to be very forgiving to changes. what can happen if necks are too tight? how can i slug the chamber to see what she actually is? any other thoughts or recommendations?
Thanks and all the best,
Don

 

[Larryh128]

Was it a custom ordered reamer or a stock reamer? A lot of stock 30 cal reamers use a .340 neck. You could be seeing some variation in the cases. I had some bulk Norma 300WSM brass that I sold after 1x firing. It was hard to find 2 cases that were the same size. The rims & grooves were all over the place.

 

[Preacher]

You can slug the chamber with Cerrosafe, and it can be had from Brownells..
Will a bullet slip back into a fired case ??
If not you really need to turn the necks until they do...

 

[jonbearman]

I would turn down all the cases so they all measure the same or you can just do 10 cases and fire them and measure to gather data and see if your group sizes get better or worse.Just make sure you turn the necks so you have at least.003 clearance to be safe in a hunting rifle.

 

[BoydAllen]

No disrespect intended, but IMO you are approaching the issue of these dimensions too casually. What sort of measuring tool are yo using to measure loaded round necks? Cerrosafe is a good tool to use to determine your chamber neck dimension, and the proper tool to measure loaded round necks, and your Cerrosafe cast is a 1" mic. that measures to .0001. Based on the results that successful thousand yard shooters have gotten, having more clearance, perhaps .003-.004, may actually give equal or better overall results, and a better margin of safety. Another tip, readers may be put off by claims of quarter inch accuracy "all day long" I know that this is a common expression on shooting forums, but experienced benchrest competitors will generally discount the credibility of shooters who use this expression. I mention this, not so much to cast doubt on your results, but as general advice to all posters. It sounds like you have an exceptional rifle, and do a good job shooting it, but I would measure what you have exactly, and depending on what that reveals, probably do some neck turning.

 

[Killshot]

I can't speak to the safety issue, above my pay grade.

But I have experienced something similar and my issue was resolved simply by chamfering the outside of the casenecks. I have a no-turn chamber with minimal clearance and some of the cases were catching on the bullet to neck junction.

Good luck.

 

[djtjr]

thank you for all of your comments.

i took .002 off and it still shot quite well. but not as good that said it was 12 deg outside with a lot of mirage off of the snow so any deviation could be not a great indicator. one group was right at .26 till i called a flyer (still touching the group but added a bullet diameter to the group. with the cold and mirage i had a hard time focusing. I am not familliar enough with these extreme conditions to know if that is normal or if it was an off day.

Boyd agree i may be approaching it too casually which is why i asked what the implications could be of being too tight. it is difficult for me to mess with a gun that shoots this well as for a hunting rig its not ofter that i have seen that kind of consistent accuracy. While i am more of a high power competitor i do mess with benchrest a bit and i too agree that .25 all day long is a stretch, i used it as a descriptor as it is a very accurate gun and of the last 6 5 shot groups the largest was .33 the rest were right at .25 or lower i know thats not all day long but for a non bench rest rifle in my hands i could not ask for anything more. i took no offense and appreciate the comment and details.

killshot thats an interesting thought ill have to cast the chamber to be sure of the safety issue but to Preachers point i cant get a bullet back into a fired case so there is likely no enough clearance.

 

[djtjr]

As a followup would taking .002 off a neck generally change a tune as messing with other components does necessitating starting load development again and building up from a lower base. i cant imagine a presume spike, but i assume changing the neck could effect tension and release and perhaps move the node a few grains either way???

Does anyone on here fine tune their load by messing with neck thickness as they would with neck tension or seating depth or is that too far even for this group (meant with respect for this group not disparagement)
thanks for the help as always.
Don