To bump 1 fired cases or not?

[Brummy Mark]

.223 tight spec chamber.

New Lapua Brass

1.750

0.248 od neck

1.451 shoulder with comparator

 

Once fired brass

1.751

0.248

1.454

 

I now have a fair few once fired cases, now do I just neck size, or do you accuracy aficianado's use a full length die set to just neck size and bump back shoulder 0.001" or 0.002"?

[minkstone]

For my 6BR, I always neck size then bump the shoulder 1 1/2 thou everysingle time using a seperate bump/body die.....it works for me.

 

 

Regards.

[DaveT]

You really are inviting the full range of 'opinions' about best practice here!

 

For myself I find that I get best results by mainly neck sizing (and work the brass much less) but I also track headspace to see if it is reducing over a series of firings (IE case shoulder moving forward) and take a decision to bump back by @ .001" from the selected point.

 

I have found that some cases will actually 'shrink' on first firing ...(IE headspace increased via brass expanding radially and shrinking in length) and then grows over a series of firings so think that tracking H'space is worthwhile.

 

I find that with my loads (relatively 'stout') I will want to 'bump' at between 5 and 8 firings to avoid hard chambering and that is across 308 Win, 260 Rem and 338 Lap mag..

 

I also find that Lapua brass is quite 'hard' in that it seems to resist too much brass growth so headspace changes very gradually.making shoulder 'bumping' a periodic exercise.

 

I know that others will say FLS every time for consistency (and that is by no means a wrong answer) ....in the end it is , once again , what works for you in your guns and with your loads which may be 'hot' or mild and affect the brass taht much more or less at each firing.

 

Why not try neck sizing & tracking headspace / keeping a 'feel' for any hard chambering and see how your accuracy goes at first?

 

You can always go for FLS if you feel that would be better after experimenting.

[Spray1Mark]

I agree with the above, main criteria is how hot the load is will determine if you have to bump the shoulder or not, we tried the chrono test and found the SD was slightly lower on every time bumped brass than on just necksized over a number of firings and essentially allowing the case to become tighter over a number of firings.

I use the body die every 3 firings but found cases were extremely tight after 1 firing during the tropical heat we had a few weeks back due to the pressure rise, I can get 5 or so neck sizings during the dead of winter before having to use the body die.

[The Gun Pimp]

.223 tight spec chamber.

What is a 'tight spec chamber'?

[gbal]

What is a 'tight spec chamber'?

Vince,

Other sporting rifles: "Wylde by Wylde"-the post by Bradders on 2 Aug had a link to an ar15barrels.com list

of eight 223 chambers....mainly throaty differences,but maybe related to these "tight specs'"?

G

[The Gun Pimp]

Vince,

Other sporting rifles: "Wylde by Wylde"-the post by Bradders on 2 Aug had a link to an ar15barrels.com list

of eight 223 chambers....mainly throaty differences,but maybe related to these "tight specs'"?

G

I was wondering what was 'tight' on Brummy Mark's chamber.

[Brummy Mark]

I did try to pm you GP, your box must be full. You bedded this rifle and may know more about it than anyone is telling me, hey ho though, I just want to crack on and get it shooting better than it is at present, perhaps this is as good as its going to be, in which case its been an expensive lesson to learn! and oh how I have learned.

[The Gun Pimp]

OK - for a start, it's essential to get your brass sized properly - full-length sized that is. Neck-sizing is for benchrest with 'fitted' necks etc - even then, I still full-length size - 'cause I like to run the gun slick, with a bolt that lifts and closes like the proverbial 'hot knife through butter'.

 

First, remove the firing-pin assembly from your bolt and run a few fired cases through the gun. Adjust your dies so that with a full-length sized case, you can JUST feel the bolt touching it - in other words, the bolt handle doesn't fully drop under its own weight. Variations will mean you won't get every case dead-on - don't worry.

 

You may find that your dies - even set up with the shell-holder touching the bottom of the die - aren't bumping the shoulder. This is quite a common problem and why I always chamber my own guns 'long' (max. spec. chamber?) - it allows me to play with bumping etc. Also remember - when you squeeze anything - you invariably make it longer! Dies can work in mysterious ways......... In other words, running the case through your die could make it longer not shorter! Brass work-hardens with firing and distorts under 50,000 psi - you won't easily reverse that with a hand loading press.

 

If your dies won't bump the shoulder, you need to turn a few thou. off the bottom of the die or grind-off the shell-holder.

 

Now you have your brass properly sized - you can work on the bullet and seating. With a factory rifle, I wouldn't seat into the rifling. Seating into the rifling came about (like most accuracy myths) through benchrest - we run light neck-tension so no matter how far out I seat my bullet, the rifling will always shove to back to 'just snugged in' - perfect!

 

If you have normal neck-tension then variations in bullets (another subject) will alter things - some may be 'hard' into the rifling, others less so.

 

I would suggest that the words 'min spec chamber' are simply 'gunsmith speak' and mean absolutely nothing.

 

Once you've got your set-up right, time to do some testing. First job is to find out what YOUR gun will take pressure-wise.

 

We do this by loading up say ten rounds - starting with the loading manual's starter load then working up in say one third of a grain increments. Fire each one - carefully looking for pressure signs - firstly, flattened primers, cratered primers, sticky bolt-lift and finally that little shiny circle on the case-head - when the brass flows into the ejector-pin hole. Now you've gone too far - shoot at this pressure and you will ruin your brass, step back a half grain or so then start your group testing.

 

Shoot three shot groups and find your best load - confirm it with five-shots. If you must play with seating depth, now is the time but be aware, with factory bullets there can be significant variations in shape which will render your work null and void.

 

A few years ago, I had the opportunity to visit a bullet making factory. The copper jackets are swaged using a solid tungsten carbide die - which lasts for just ONE eight hour shift! You can imagine the variation in bullets between one made at the beginning and one at the end of the die's life. Then consider that there are a dozen machines spitting out bullets into a vast hopper - which are then scooped into boxes. Variations? You bet! This is why we use hand made bullets for benchrest.

 

Sorry it was so long

 

Vince

[Brummy Mark]

Thank you for your comment Vince.

I have already done as you suggested regards running cases through the chamber with firing pin removed. I use Sinclair gauge to check max coal.

I have a chronoed and sensible velocity arrived at, and have tweaked the powder to smallest group, now tweaking jump to most consistent group to POI, and had 22.8 N140 at .040" out of lands and 22.9 0.030" out of lands as best groups. Have now loaded up 5 rounds of N140 at 22.8, 22.9, 23.0 and 23.1 all at .035" out of the lands. This Bartlein 1.725 twist barrel doesn't have a lot of freebore though.

 

Only things yet to change are powders and primer.