How to gauge bullet depth off rifling?

[Scooter1942]

Hello all. I've been away for about a decade and recently I've found myself shooting long range and precision again for recreation instead of competition. I just started the journey into 6.5 CM and purchased a 20" barrel for my LMT MWS. I've been looking at load data for the 6.5 CM and I keep seeing that people are setting the bullet back to about .020 off the rifling.

So my question is...how would one go about determining this seating depth with a gasser...or even a bolt gun for that matter? My initial thought is to simply load a bullet as long as will fit in the magazine and then drop the bolt on it, allowing the bolt to seat the bullet. Then back off of that bullet length by .020. Sounds crude, but would it work?

 

[Baron23]

Lock‑N‑Load® O.A.L. Gauges & Modified Cases - Hornady Manufacturing, Inc

Lock-N-Load® O.A.L. Gauges Regulating seating depth and the resulting jump to the rifling is widely regarded as fundamental to improved accuracy. A few thousandths of an inch change between the bullet and the rifling can make the difference betw...

www.hornady.com

 

[remaction]

For a bolt gun, I like the Alex Wheeler method. View video here:

Videos | wheeleraccuracy

www.wheeleraccuracy.com

 

[DubfromGA]

Nosler reloading manuals used to detail an effective method that didn't require any special tools other than your reloading calipers & a Sharpie pen.

 

[wcberry]

Or

 

[JR1200W3]

Measuring the lands on a gas gun is more difficult. I'd recommend a tool made for the task. Both Hornady and Sinclair sell tools.

 

[Pharmseller]

Sharpie method here.




P

 

[Scooter1942]

Care to describe that method? Curious.

 

[Pharmseller]

Here’s how I was taught:

Overview: You’re marking up a bullet so that the lands make a mark when the cartridge is chambered. What you’re looking for is a square mark on the bullet, so that the bullet is just kissing the lands. Not jammed.

Have a bullet puller handy.

Using sized but unprimed, unpowdered brass, seat the bullet long. Make sure you have a cleaning rod handy in case the bullet gets stuck in the lands. Make sure the cartridge length does not exceed your mag box or magazine length, unless you plan on loading singly.

Measure the oal.

Using a Sharpie, color up your bullet of choice. A little forward of the ogive, then back a little way. Let the ink dry.

Chamber the dummy cartridge. If it’s clearly too long, remove the dummy, seat a little deeper then re-ink. Measure again, then chamber the cartridge.

Repeat the process until the mark left by the lands is square. The bullet isn’t jammed into the lands, so no worry about pulling the bullet out when you unload.

Measure the oal (I measure off the ogive with a comparator) and try again using a different bullet (same model and weight, of course) and brass, just to confirm.

That‘s you’re max with that bullet in that rifle.

Might be easier ways, but this one works for me.





P

 

[Aftermath]

Nosler reloading manuals used to detail an effective method that didn't require any special tools other than your reloading calipers & a Sharpie pen.

Sharpie method here.




P

I am old. My mentor had me using candle sooted projectiles.

 

[Aftermath]

Here’s how I was taught:

Overview: You’re marking up a bullet so that the lands make a mark when the cartridge is chambered. What you’re looking for is a square mark on the bullet, so that the bullet is just kissing the lands. Not jammed.

Have a bullet puller handy.

Using sized but unprimed, unpowdered brass, seat the bullet long. Make sure you have a cleaning rod handy in case the bullet gets stuck in the lands. Make sure the cartridge length does not exceed your mag box or magazine length, unless you plan on loading singly.

Measure the oal.

Using a Sharpie, color up your bullet of choice. A little forward of the ogive, then back a little way. Let the ink dry.

Chamber the dummy cartridge. If it’s clearly too long, remove the dummy, seat a little deeper then re-ink. Measure again, then chamber the cartridge.

Repeat the process until the mark left by the lands is square. The bullet isn’t jammed into the lands, so no worry about pulling the bullet out when you unload.

Measure the oal (I measure off the ogive with a comparator) and try again using a different bullet (same model and weight, of course) and brass, just to confirm.

That‘s you’re max with that bullet in that rifle.

Might be easier ways, but this one works for me.

P

I was taught to strip the bolt of the ejector...nothing but the bolt face on that cartridge base. Also, muzzle up so gravity is working for you to keep the bullet out of the lands inadvertently. It doesn't have to be vertical, just sloped up rather than down.
AR's you can also just pull the barrel. This makes for some real touch sensitive and IMHO accurate measuring.
Rather than measure the OAL, I go for the cartridge base to ogive. The only time I worry about the OAL is whether it works in whatever magazine I need it to. I do check to be sure of magazine function. If seated to the ogive touching the lands is too long for the magazine, I still know that CBTO touching the lands dimension and write it down in my book. If the seated to ogive touching the lands is too long for the magazine, I load 5 dummy rounds right at max length for the magazine and label them as such. Then I start pushing them deeper until they feed from the magazine (via hand, of course) reliably. Then I measure CBTO of those dummy rounds and set up the dies as such for the start of load development. I can then figure out how much jump I have and start exploring options like charge weight or even deeper seating or whatever makes it shoot more gooder.