Determining seating depth

[mdevers49]

I'm a relative newbie to reloading and could use some help regarding bullet seating depth. I have a custom built blueprinted remington 700 long action (it started life as a 300 win mag) in 308 with 26in match barrel. Assume the resizing die is set correct. I have tried several things to help with seating depth. Among them, I have tried to polish the bullet and look for the lands marks and keep seating deeper until I can't see any. problem here is the marks are still showing when the oal goes well under 2.80 in. This would be ok maybe but it causes compressed loads even with the lighter charges(around 42-43 grs). I have tried the remove the firing pen method and it looks and feels good until the firing pin is back in and then it seems jammed again. My best load so far seems to be 44.5 gr of imr 4064 with a 175 gr Sierra match king. So I'm trying to find the best seating depth for that now. Any thoughts?

 

[mike in co]

there a varity of tools available to help measure seating depth. the stony point tool with one of thier cases will get you there.......
you can modify ont of your fired cases and use it.....
the looking for the marks on the bullet is a very judgemental call...the stony point tool will give a specicic number you can repeat in loading.
mike in co

ps there is not requirement for the oal to be longer than 2.8......its possible the throat is short to keep the loaded round to std 308 win deminsions.

did u build on a long action to load long ?? does the gunsmith know htis ??
just does not sound right.

 

[Larry Elliott]

I can't see rifling marks on a polished bullet either, but find that marking the bullet with a black marker allows the rifling marks to be seen easily.

As Mike says the 2.80" OAL for the .308 is the max OAL set for the .308 and a custom chamber can allow either longer or shorter OAL depending on the reamer that was used to cut it. An OAL shorter than 2.80" may be advantageous because it will allow the bullet to be seated out as the throat erodes (chasing the rifling) to maintain accuracy.

The advantage of starting your loads with the bullet showing equal width and length rifling marks, is that as you seat the bullet deeper pressures won't increase. If you start the bullet seated deeper and move it into the rifling pressures can increase enough to cause difficulties.

The best way to adjust OAL and seating depth is to first determine where the bullet is giving equal width and length rifling marks, then using a bullet comparator (sold by Sinclair Int'l, and others) that measures the distance between the head of the case and that point on the bullet ogive where it contacts the rifling. This will vary from one bullet type/make to another btw. Adjust your seating die to give a shorter comparator length of 0.010, 0.020, and so on until you reach the seating depth that produces the best accuracy. Overall length will vary from bullet to bullet from the same lot of bullets with no adjustment of the seating die, but the comparator length will be much more consistent.

As Mike says building a .308 on a .300 Win Mag bolt face sounds a little strange. It'd seem a .30-06, .270, or .25-06 action would have been a simpler action for a .308, and most shooters use short actions for the .308 with no difficulties except maybe for feeding loads longer than 2.80" from the magazine. Whatever.

 

[Boyd Allen]

To make rifling marks much more visible, wrap some 0000 steel wool around the bullet, (in front of the case neck) apply pressure to the " bullet sandwich" with your thumb and the side of your curled index finger, and spin the round a couple of revolutions holding the pressure on the bullet. This will make tiny scratches around the bullet, that will have no effect on its function, and which will make rifling marks very visible.

 

[Vern]

Boyd's method works fine.
I also will use a candle and smoke the bullet. It seems to work better for me.
Make sure its a dummy round.

 

[mike in co]

boyd's pic is a clear example of what i meant with my comment on reading marks.
boyd's marks are clearly shorter( front to back) than they are wide( circumfrence)...........yet another person said they should be the same....
my pet peave with the issue is the inexactness of the process for the novice. no way to tell how close one is to square( same length as width)......how does one measure this way ??

iastoney point tool is repeatable with practice.......it is a tool and is only as good as the user....

mike in co

 

[Boyd Allen]

Mike,
None of us are born knowing this stuff. What we know, we learned from discussions with other shooters and by trying out various types of equipment and procedures, and making judgments about what we like for various situations. I was not trying to say what method should be used, but simply sharing a way to make marks on a bullet easier to see. I assume that because this is a hobby that one of the small pleasures that we all enjoy is doing a little research and making up our own mind about what to buy or do. I was just trying to add a little info. to the pile, not telling him which was better, or what to do. If you don't like marks, by all means, don't use them. It is after all, your hobby.
Boyd

 

[Charles E]

I have tried to polish the bullet and look for the lands marks and keep seating deeper until I can't see any. problem here is the marks are still showing when the oal goes well under 2.80 in. This would be ok maybe but it causes compressed loads even with the lighter charges(around 42-43 grs).

Use empty, unprimed cases, and if you can, minimally sized to start. At some point, you should see the marks on the bullet go away. Then seat the bullet a bit longer, and again, at some point, you should see the lands begin to engrave the bullet.

When I do this, I do use calipers and measure the OAL before and after chambering. It's just a check. A variant on the above is to take a lightly neck sized case, seat the bullet long, and chamber it. The chambering itself will push the bullet back into the case neck. Now, with regular neck tension, check this by seating a bullet about .010 longer in OAL. You should now see some engraving marks on the lands. If not, something is awry. Measure the OAL before and after, to make sure that what the lands are showing you doesn't include some residual pushing of the bullet back into the neck.

In and out, in and out. At some point, you'll find a spot you consider "just touching." Whether it is in an absolute sense doesn't matter, it is your reference point. Now you can begin seating bullet farther into and farther off the lands, to see the effect on accuracy.

If, as you say, any seating of the bullet off the lands compresses the powder, that may limit how much jump (off the lands) you can use. That depends on neck tension a bit, too -- if you run a lot of tension, the powder won't push the bullet back out as quickly as if you use little neck tension.

If you're begining to think that all these measurements are a bit relative, they are. Both in terms of what you can do (how much in/off the lands), and what the numbers themselves mean. Remember, what counts is repeatability, not some magic number that can be shared. This is why most of us feel the Stony Point or other such device is a waste of money. To each their own. It works, but so do other methods.

As to your particular problem, that you fill the case so full that any attempt to jump the bullet isn't effective, the tried & true solution is to throat out the chamber just a bit. This is one place where a magazine fed rifle may have different requirements than a singly-loaded rifle. If the rounds have to be fed from a magazine, that's another constraint. But I'd bet a long action in .308 will give plenty of room.

Once you get the hang of it, get a technique going, it doesn't take all that long to establish your "just touching" point. I just went through that for a new chambering; it took me about 15 minutes. If I remember right, the first time it did this, it took me over an hour.

Good luck with it, seating depth -- the amount of jump/jam -- is something that matters. It can also vary from barrel to barrel, or from bullet to bullet, within a barrel.