OAL Gauge - Jump

[HALODIN]

Hello,

Just a quick question ~ I'm just about to seat some bullets and I've tried out my OAL gauge for the first time. I've taken 40 different readings against my AI-AT .260 with fire formed brass (2 different cases) and 3 different bullets and thought I'd just check here to see what people recommend in terms of a new COAL. I've read you should set the jump to 0.020 (20 thou) off the lands to start with and work your way back, so with that in mind, are these new COAL's suitable?

2.825
2.805
2.885

Does that sound reasonable? More info below if it helps.

Cheers,

Craig.

Bullet: 139 Scenar Lapua
COAL: 2.795
Powder: N560
Powder range: 39.6 - 44.1 (Currently loading with 42gr)
Velocity range: 2311 - 2586

http://www.lapua.com/upload/downloads/b ... ed9eng.pdf

OAL Gauge range: 2.842 - 2.849

AVERAGE COAL .260 AI-AT = 2.845
PREVIOUS COAL: = 2.795
DIFFERENCE: = 0.050

2.8430
2.8430
2.8450
2.8450
2.8440
2.8440
2.8435
2.8435
2.8435
2.8460
2.8460
2.8480
2.8460
2.8460
2.8435
2.8440
2.8445
2.8445
2.8445
-------
2.8490
2.8445
2.8470
2.8455
2.8480
2.8465
2.8455
2.8475
-------
2.8475
2.8445
2.8445
2.8465
2.8455
-------
2.8450
2.8465
-------
2.8475
2.8440
2.8460
2.8490
2.8480
2.8445
2.8420

[HALODIN]

Sorry that should have been...

2.825
2.805
2.785 (not 2.885)

[The Gun Pimp]

Bit meaningless to me.

Your COAL is the distance from the face of YOUR (closed) bolt to the tip of YOUR chosen bullet when 'just touching' the rifling - bearing in mind that bullet meplats are 'ragged' and will cause slight variations.

No one can tell you what your COAL is - bit like asking us what your collar size is - absolutely no idea!

But, starting 20 thou. off the rifling is a good idea.

[HALODIN]

Thanks for the reply, although I don't follow why it's meaningless. My COAL are those 40 measurements, albeit with an open bolt. OK perhaps I'll be more specific.

1) Whilst I should start 20 thou off the lands, what increments are worth experimenting with to improve accuracy? [2.825, 2.805 and 2.785] or [2.825, 2.815, 2.805, 2.795, 2.785] or some other increment?

2) Changing the COAL has implications for pressure, so how far can you go with this? The recommended COAL from Lapua in the link above was 2.795, how many thou each way is still considered safe with the other details provided above?

[M99]

Start with a standard length and find a charge weight that works in your rifle. Then fine tune it by adjusting the seating depth both in and out (if you need too)
I now almost exclusively use the OCW method with a tweak or 2 to find my most suited load with a particular powder. I always start with the bullet seated to the recommended depth. Then if the load needs to be tweaked I will change seating depth/coal.

[Dougan]

My understanding of measuring COAL to minimise jump is that you use the guage to measure the point at which the ogive of the particular bullet you intend to use touches the lands of the rifle, and then seat it 20 thou (or whatever you chose) deeper...thus getting the bullet as close to lands (minimising the jump) as possible without it actually touching the lands.

You will need a .260 comparitor insert for you calipers to measure from the ogive (which vary with bullet type) to get both the measurement when using the gauge, and for checking the OAL of you finished rounds.

Also, yes seating depth affects the pressure - If you have an OAL less than the manual, then all things being equal (bullet type ect.) the pressure would (theroretically) increase, and if you have an OAL longer than the manual then the pressure would decrease...

...if you're only playing with 20 thou or so then the pressure differences will be small, but also bear in mind that seating a bullet a long way out from recommended will also lessen the neck tension; which can also affect performance.

[The Gun Pimp]

Thanks for the reply, although I don't follow why it's meaningless. My COAL are those 40 measurements, albeit with an open bolt. OK perhaps I'll be more specific.

1) Whilst I should start 20 thou off the lands, what increments are worth experimenting with to improve accuracy? [2.825, 2.805 and 2.785] or [2.825, 2.815, 2.805, 2.795, 2.785] or some other increment?

2) Changing the COAL has implications for pressure, so how far can you go with this? The recommended COAL from Lapua in the link above was 2.795, how many thou each way is still considered safe with the other details provided above?

YOUR COAL is measured using YOUR chosen bullet in YOUR rifle. The COAL recommended by Lapau is meaningless for your purposes - i.e attempting to get the best accuracy out of your rifle.

Once you have YOUR COAL measurement, you can use it to load your ammo. A good starting point would be 20thou. off COAL. i.e - the bullet jumping 20 thou.

Adjust the powder load to give you the best result (smallest grouping) and then and only then, experiment with seating-depth.

After 20 thou. off the rifling, try 10 thou off. Is the group smaller? If you go close to the actual COAL measurement - i.e. the bullet touching the rifling - this is often the worst place to be. Reason? Bullets vary slightly and you will find some rounds 'just touching' others a couple of thou. off and some a few thou. into the rifling. Worst possible scenario.

You could try shoving them into the rifling 10 thou. but personally, I wouldn't. I only load into the rifling for benchrest competition where we run very low neck tension - so that the bullet is pushed back into the case on closing the bolt and each round ends up exactly the same. With mass produced bullets and heavy neck-tension, you can't easily do this. Also, if the RO calls a 'cease fire' there's a good chance you could pull a bullet when you open the bolt on an un-fired round and fill the action with powder - shoot over!

Will loading into the rifling raise pressure? Yes it will a little and 'a little' could be too much if you are close to maximum so, if you decide to go into the rifling, back off half a grain.

Halodin - if I'm not making sense, please e-mail me on vinceb@6ppc.fsnet.co.uk and I can take you through it more thoroughly and give you a few tips on COAL measuring and handloading for accuracy.

[HALODIN]

Yep that's pretty much where I am, I have run through load development and found what I deem to be a pretty reasonable load. I now want to play with the seating depth...

I've never heard of OCW and whilst I've only read the summary page, it sounds fascinating. I like detail so this is right up my street. I've always thought that saying "find a round that your rifle likes" is really unscientific and there must be more to this, it's just we don't understand it as well as we should. I'll read some more, thanks for the tip-off.

http://optimalchargeweight.embarqspace.com

Start with a standard length and find a charge weight that works in your rifle. Then fine tune it by adjusting the seating depth both in and out (if you need too)
I now almost exclusively use the OCW method with a tweak or 2 to find my most suited load with a particular powder. I always start with the bullet seated to the recommended depth. Then if the load needs to be tweaked I will change seating depth/coal.

[HALODIN]

Thanks. Yes that reaffirms my understand as well, but I just needed to check. I have my .260 comparitor which I plan to use on this batch. Whilst I might not be able to improve the accuracy of the rounds, it's as much a science experiment as anything. I just wanted to apply a bit more science to my reloading.

My understanding of measuring COAL to minimise jump is that you use the guage to measure the point at which the ogive of the particular bullet you intend to use touches the lands of the rifle, and then seat it 20 thou (or whatever you chose) deeper...thus getting the bullet as close to lands (minimising the jump) as possible without it actually touching the lands.

You will need a .260 comparitor insert for you calipers to measure from the ogive (which vary with bullet type) to get both the measurement when using the gauge, and for checking the OAL of you finished rounds.

Also, yes seating depth affects the pressure - If you have an OAL less than the manual, then all things being equal (bullet type ect.) the pressure would (theroretically) increase, and if you have an OAL longer than the manual then the pressure would decrease...

...if you're only playing with 20 thou or so then the pressure differences will be small, but also bear in mind that seating a bullet a long way out from recommended will also lessen the neck tension; which can also affect performance.

[HALODIN]

You're making complete sense, although I wouldn't mind taking you up on some of those tips if you don't mind. I don't know anyone who shoots benchrest, so I'm curious how deep the rabbit hole goes... Yes Lapua's COAL was just a starting point, which is why I'm experimenting now, I just want to see what's possible.

Thanks for the pointers, I've taken them all onboard and for the moment at least, I'll start 20 thou off the lands and move away from the lands. I was speaking with someone at our club today about it and they thought 5 thou increments would be smallest they'd consider testing. I think I'll try 10 thou and if the difference is notable, I'll try and fine tune it further.

Do you mind if I fire you an e-mail?

Cheers,

Craig.

YOUR COAL is measured using YOUR chosen bullet in YOUR rifle. The COAL recommended by Lapau is meaningless for your purposes - i.e attempting to get the best accuracy out of your rifle.

Once you have YOUR COAL measurement, you can use it to load your ammo. A good starting point would be 20thou. off COAL. i.e - the bullet jumping 20 thou.

Adjust the powder load to give you the best result (smallest grouping) and then and only then, experiment with seating-depth.

After 20 thou. off the rifling, try 10 thou off. Is the group smaller? If you go close to the actual COAL measurement - i.e. the bullet touching the rifling - this is often the worst place to be. Reason? Bullets vary slightly and you will find some rounds 'just touching' others a couple of thou. off and some a few thou. into the rifling. Worst possible scenario.

You could try shoving them into the rifling 10 thou. but personally, I wouldn't. I only load into the rifling for benchrest competition where we run very low neck tension - so that the bullet is pushed back into the case on closing the bolt and each round ends up exactly the same. With mass produced bullets and heavy neck-tension, you can't easily do this. Also, if the RO calls a 'cease fire' there's a good chance you could pull a bullet when you open the bolt on an un-fired round and fill the action with powder - shoot over!

Will loading into the rifling raise pressure? Yes it will a little and 'a little' could be too much if you are close to maximum so, if you decide to go into the rifling, back off half a grain.

Halodin - if I'm not making sense, please e-mail me on vinceb@6ppc.fsnet.co.uk and I can take you through it more thoroughly and give you a few tips on COAL measuring and handloading for accuracy.