Seating depth adjustment

Using a Hornady comparator, I've found indicated base to ogive variations in a batch of 142 SMKs of 10 thousands / 0.010. This results in differences when I measure max cartridge base to ogive in my rifle chamber for each bullet length.

If my optimal verified seating depth with the bullet that measures 0.776 with max cbto of 2.265 is 15 thousands off the lands at 2.250 cartridge base to ogive, does this mean that the bullet that measures 0.765 and measures max cbto at 2.258 should be set back to 2.243?

Or do I have to test each bullet length? Or am I over thinking this?

Thanks

 

Are you sure your chambers clean prior to measuring? .010 variations is a lot.

 

I'm new to this cbto thing myself, but I'm thinking you need to sort your bullets first using compairitor.
Now you can seat individual lots to desired cbto length.
Somebody correct me if I'm wrong.

 

I measure my bullets and sort them. I then will adjust my seater to give me the same CBTO measurement.

 

Once setting your base to ogive setting giving you a 15 thousand jump is the most important , some bullets in different lengths maximum 10 thousand but grain weight is accurate , 10 thousands maximum deeper in the case wouldn't make any difference down range . How are you sizing your brass , bolt face to datum , how much wiggle room ?

 

Perhaps you're over thinking this. While .010" variation is more than I've seen even in quality hunting bullets (i.e. Sierra and Nosler) if you seat .020" off the lands you should be fine for any variation in bullet ogive. Also you want to have enough bullet tension* and make sure the round will fit the magazine.

I'm not familiar with the Hornady Comparator tool but perhaps the variation is due to the measure method you are using. I use a Frankfort Arsenal type tool method (homemade version) which is quite simple, inexpensive and reliable.

* I like at least one bullet diameter seating depth for hunting reloads.

 

IMO ....the bulk of the difference should be in the neck. It may affect your load a fuzz. but ? I use the Hornady comparator's to get a reference point for where my bullet is to the lands, but I don't put much faith in it for measuring bullets accurately every time. I myself chose to buy better bullets, and it's worked out well for me so far.

 

only variations in shape of ogive will effect seating base to ogive. given that the ogive shape is consistent longer bullets will just seat deeper in the case. load them and check base to ogive. they should be pretty consistent with a good.seating die. then shoot them and see what the target tells you.

 

As grovey and Richard pointed out, bullet base-to-ogive variance does not affect seating depth. Only bullet length variance between the point at which your comparator insert contacts the ogive and the point on the ogive farther out toward the meplat at which the seater die stem contacts the bullet will cause seating depth variance. It's certainly possible that bullet BTO variance may be linked to nose variance between the caliper insert and seating die stem contacts, but it doesn't have to be. The easiest solution is to use something like Bob Green's comparator tool (or similar), which allows you to sort bullets by length variance in the region between caliper insert and seater stem contact points.

 

Sounds more like a weird batch of smk
I had to quit using 100 boxes order 500 and start over

 

so you are getting variations of 0.010 checking seating depth with different bullets from the same lot in your rifle chamber?

is your chamber spotlessly clean. no carbon ring?

what method or tool are you using to measure to the lands?

or is this variation just sorting bullets. if it bothers you a lot you can always sort them.

i would load a few and measure after seating or use a stoney point and check several of different length for distance to the lands. if there is a large variance then it is obvious that it is an inconsistent ogive and i would pick a different bullet

 

I'm just wondering if you guys are finding an optimal distance off the lands and loading to that, or a fixed cartridge base to ogive length. I definitely have 142 smks with 10 thou base to ogive variation, although usually not in the same 500 box.

 

That depends on single shot or magazine feed.

 

Have you checked your seating die to insure that the seating stem is not pushing on the tip (meplat) of the bullet? I've had to modify several of mine by drilling because of seating variation. Those long, slender bullets frequently don't mate up well to a standard seating stem.

When beginning to load a new, heavier and longer bullet, i'll pull the seating stem and switch it out if it doesn't fit the bullet solidly. You can order new stems from your die manufacturer that are made for high BC bullets. Many times shipped free by customer service.

 

I sorted a box of Sierra 190gr HpBt match bullets 500 count by bearing lengths which resulted in a 0.020” difference in lengths.

I seat 0.005” from just touching the lands. I would not have a consistent seating distance without sorting.

 

VLD bullets may need a different seating stem but only on VLD . When seating from ogive to base or bolt face , slight length differences shouldn't effect the ogive measurement . I could see different case to datum measurements effecting the ogive . Let's say your case measures .001 and the other at.002 from base to datum , with a bullet hitting the lands with the case at .001 , the other case at .002 will be a .001 jump on the ogive with a case at .002 . Will be the reverse if starting with the .002 case . We're not measuring tip to base OAL measurements.

 

i use bergers and have never seen that much variation. now correct me if i am wrong but there are two things that can vary in this situation.

one is the consistency of the curve of the ogive. this would cause your comparator and your seating stem to catch the curve of the ogive at different points with different bullets. this would definitely effect seating depth.

the other would be bullets longer or shorter from base to ogive. in this case the longer bullets would just seat deeper into the case not effecting seating depth. now they may have a longer bearing length that might effect accuracy. takes two comparators to measure bearing length with a boattail bullet.

loading them and measuring base to ogive is the only way i know to tell which one it is. i would be curious what you find out. or really you could use a stoney point and find the lands with several long bullets and several short bullets and compare overall length.

 

If my optimal verified seating depth with the bullet that measures 0.776 with max cbto of 2.265 is 15 thousands off the lands at 2.250 cartridge base to ogive, does this mean that the bullet that measures 0.765 and measures max cbto at 2.258 should be set back to 2.243?

That’s a question I have also...I’m having trouble wrapping my head around a good answer in all those responses though. So far, I’ve just loaded to my desired CBTO... knowing that some bullets may be pushed down further into the case than others, though.... right?? Makes me doubt my technique (and ability) that’s for sure!

 

So far, I’ve just loaded to my desired CBTO...

now did you have to adjust your seating die for each bullet to get your desired CBTO?

 

Once your cases are sized the same your seating die shouldn't be moved the receive the same measurement from base to ogive .