Gunsmithing understanding and measuring throat length

[jayd4wg]

I'm reading on other forums that my particular brand and model of rifle has a propensity to have an overly long throat. I understand that this can be a detriment to accuracy in some cases and would like to understand why.

Secondly, what is the best way to measure what I have, given what I have. A Lee Loader, a bunch of spent brass, and will have some bullets shortly. Can i place a bullet into a resized case, flat based bullet mounted backwards and then chamber the round and extract, measuring for total lenth with caliper? I guess if done right this would give an indication as to the total length of the chamber, to the lands? Assuming this length SHOULD be a touch over the total max OAL of 2.80 inches for the .308, how much past this is acceptable, and how much would warrant a trip back to the factory?

 

[Hateca]

Re: understanding and measuring throat length

Depends on the bullet you're going to use.

If this is a factory rifle you can forget about sending it back to the factory because you think the throat is too long, there're all long.

.308 likes to jump a little so don't get twisted all up about how long the throat may or may not be. Shoot GM that is 2.80 and if it does well then why would you need to worry about the throat. Worry about that when you get a custom barrel.

There are lots of ways to measure it one way is how you describe. Another is with a Stoney Point tool. But these only give you a rough idea. You want to know for sure you would need a chamber casting done.

 

[dinc]

Re: understanding and measuring throat length

Jason, Randy (HateCA) is a god. Listen to him.

I have a .308DM PSS. Magazine length handloads are limited to 2.8" and usually shoot about 3/4 moa. Seated to just touch the lands, currently about 2.93", it shoots a little better. Like maybe 0.1" better. This is shooting nice and slow with my best 'benchrest technique'. Unless you are using a rear bag and taking your time, you will never notice the difference.

The classic method for finding land length is to seat a bullet in an empty case (no powder or primer -a fired case will usually hold the bullet tight enough for this measurement. If not you can partially neck size a case, just what your Lee Loader does). Then smoke the bullet over a candle flame to get it all sooty. Gently chamber and extract the round. Look closely at the bullet and you should see where the lands marked the soot. Make small adjustments to seating depth until you can just see the copper through the black. This is your seating depth to just touch the lands. Adjust accordingly for best accuracy. Some will alternatly color the bullet with black sharpie.

Your best bet. Load a bunch of rounds to 2.80" and go shoot. In the end the time spent shooting will give you more hits and/or a better score than the time spent messing with oal.

 

[jayd4wg]

Re: understanding and measuring throat length

Roger that guys. I hate when I make a mountain out of a mole hill. truth be told I have MUCH to work on still, and tweaking the OAL of my reloads I can't make yet is not one of them.

I went out a week ago friday for a red tag deer hunt - took a poke at a 350 yard big momma doe. I mentally doped at 10 inches low (zero @ 200) and while watching her and the other does I took note that the bottom of my duplex reticle at max zoom was about 12 inches. SO i figured that as my POA. When it was time to take the shot, I promptly put that new POA on her back and squeezed. Twice. First round exploded a rock behind her and my buddy thought I hit her with a clean passthru. She reacted weird though and ran hard but not panic run...just a "what the fark was THAT?!?!" kind of run...while she was trotting away I took the second poke at her and this time i had a much better rest and weld - I watched this round hit behind her again and that is when I realized what I did. Nothing like a double holdover. I'm a dope when it comes to my dope - I need to get a mildot on this thing and move on. It's been a long time since I used a duplex reticle and just made a rookie mistake.

Anywho...thanks for the input.

I'll measure it just to know...i'm a tinkerer at heart and kinda anal about certain things. I'm also a cheap bastard and that is why i'm not running more costly hardware. That said, this rifle is showing promise - 1MOA with factory loads of bipod and a rear bag (or fist) and I'm not at all displeased with the trigger or the way the gun works...for the first 5 rounds. after the sporter weight barrel heats up, all bets are off cus it's pillar bedded. I may take out the tabs in the stock to freefloat...but right now I'll work on ME, some reloads, and shoot slow.

Happy shooting