Safe or Sorry? Lapua brass in 20 Practical...

[chop house]

(spoiler: i already pretty much know the answer). my loaded round neck o.d. is 0.2315. reamer print says neck i.d. is 0.233. did some cautious load development at 70F and necks measured 'no' expansion, new bullet not 'fit' in, and fired i.d. measures just at 0.205. brass freshly annealed but i may have been conservative on time. at 23.4 grains of lt-32 with 32 vMax chrono'd 3908 average, which i felt was a little high for the charge - although the case head, extraction and primer did not indicate pressure. bbl is x-cal on savage action with the usual enhancements.

so i know i should turn the necks on this small lot of lapua .223 match brass - already sized down to .20, but here is the groaner. this group popped up during load verification and the ones alongside were not bad either:

 

[strut64]

(spoiler: i already pretty much know the answer). my loaded round neck o.d. is 0.2315. reamer print says neck i.d. is 0.233. did some cautious load development at 70F and necks measured 'no' expansion, new bullet not 'fit' in, and fired i.d. measures just at 0.205. brass freshly annealed but i may have been conservative on time. at 23.4 grains of lt-32 with 32 vMax chrono'd 3908 average, which i felt was a little high for the charge - although the case head, extraction and primer did not indicate pressure. bbl is x-cal on savage action with the usual enhancements.

so i know i should turn the necks on this small lot of lapua .223 match brass - already sized down to .20, but here is the groaner. this group popped up during load verification and the ones alongside were not bad either:View attachment 1445400

WOW.

 

[chop house]

running at those reduced clearances means 0.0005 difference on neck thickness has outsized affect on pressure. as witnessed by primer flattening:

 

[Kracken McGraw]

Have you got a sine wave on the case neck?
The bullet having to fall in the fired case neck gets blown out of proportion on the internet. IMO.

 

[chop house]

Have you got a sine wave on the case neck?
The bullet having to fall in the fired case neck gets blown out of proportion on the internet. IMO.

talking can not push in, let alone falling in... i pushed one in part way, had to use a kinetic puller to get it back out.

 

[Ggmac]

I shot a 6.5 wildcat that the case would not accept a bullet after firing for years . I did get to chamfer the necks which made the bullet almost enter under its own weight . It shot amazing, pressure signs were not visible, neither case head expansion or primer flattening. Just keep an eye on the spike that may happen when you change ANY of the components.

 

[Kracken McGraw]

talking can not push in, let alone falling in... i pushed one in part way, had to use a kinetic puller to get it back out.

Yep, if you can’t push another loaded round into the fired case and pull it back out fairly easy, sounds too tight.

 

[L.Sherm]

Problem with that tight of clearance is leaves nothing for the start of a carbon ring which takes not that many rounds to start developing

 

[L.Sherm]

From my memory every single 20 cal round that uses Lapua brass they need .235 for nk clearance if you want no turn

 

[chop house]

From my memory every single 20 cal round that uses Lapua brass they need .235 for nk clearance if you want no turn

yup, that is what the math says to get 0.0015 per side.

 

[jepp2]

my loaded round neck o.d. is 0.2315

What are you using to make this measurement?

My loaded 20 P rounds are slightly less when using Lapua brass. I haven't turned the necks yet for my 0.233" chamber, but I'm not as comfortable as I would like using them.

Regarding your group size, you do realize in general better accuracy is normally achieved when there is adequate clearance between the chamber and the case neck.

 

[Ggmac]

What are you using to make this measurement?

My loaded 20 P rounds are slightly less when using Lapua brass. I haven't turned the necks yet for my 0.233" chamber, but I'm not as comfortable as I would like using them.

Regarding your group size, you do realize in general better accuracy is normally achieved when there is adequate clearance between the chamber and the case neck.

Usually. Every once in awhile, one will come along and make us rethink everything.

 

[searcher]

Using that same bullet, brass and powder, I run 23.6 grains for 3,825 fps out of my 26" Hart-barreled 1-11 twist A/R's. So you are running around 82 fps faster with 2/10th less powder. That 2/10 grain should be good for about 20-30 fps. So, figure 100 f.p.s. different taking that into consideration. Given you are not running a gas gun, may have a longer barrel, barrel differences in tightness, etc., I'd say your 100 fps is no cause for alarm in respect to the tightness of the neck being a problem. If it was truly tight with no space, your pressures would be much higher. That you are getting such a fantastic group, that further leads me to believe there is clearance as you note - because when there is no clearance, the minute differences in brass neck thickness (even on turned necks) will cause such erratic pressure spikes that such a group with that condition would be pretty unlikely. It would also show on your chrono if you chrono all your work-up loads. That all said, I'm assuming you are running turned necks. So taking off another .001" to .002" would be easy insurance. If you haven't turned them, these .20's shoot really well with them as compared to the improvement on the larger calibers. If all you intend to run is the V-Max 32, the carbon ring should not be an issue as the bullet is too short to reach it (if memory serves me correctly). The 40 V-Max will definitely enter the "ring neighborhood". If you were to switch to other flat-base 32's, they lack the mild boat tail the V-Max has - again putting you in carbon ring area. Those V-Maxes and LT-32 work in every 20P I have shot. That group is no doubt helped by the near-zero tolerances you have at the neck. If you only shot a few rounds here and there - I'd be inclined to leave things as they are as long as you are shooting only the 32 V-Max - but I'd be very keen to any signs of a tight neck, like unexplained high shots, smoked primers, or ANY resistance closing the bolt. Because I load up thousands of loads at a time for a squirrel shoot, I don't have time to monitor my brass that closely - so I run with more clearance than you are presently.

 

[chop house]

I'd say your 100 fps is no cause for alarm

So taking off another .001" to .002" would be easy insurance. If you haven't turned them, these .20's shoot really well with them as compared to the improvement on the larger calibers.

they were unturned and while i am used to seeing about 0.0135 on lapua necks, some of these were over 0.014. so indeed i took a small lot of about 20 cases and turned them by hand (i have a hornady 'machine' but did not have a #16 shell holder). i could not hold the tolerances i like but they are all between 0.0125 and 0.013 now. i saved the pieces that gave me those good groups, and those loaded rounds are right at 0.232 minus a hair. the ones i turned are at just under 0.230. i will shoot them side by side and report back.

btw, thanks for the very detailed reply.

 

[chop house]

What are you using to make this measurement?

i have a variety of lower end tools. i have a blade mic, a tubing mic, a 'normal' mic, inside mic, calipers, surface plate and indicator, and a sort of mandrel jig with a dial indicator where i can rotate the case and also move it in and out to check thickness both around and along the neck. i don't have 0.0001 resolution but i can pretty much tell to 0.0005.

these cases were not fresh out of the box, so some of their history is lost. brass may have flowed to the necks from previous use.