OAL Variation / Tolerances

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Davidf

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This isue has frustrated me for years. Last night I was reloading some .223 target loads (68 gr Hornady HPBT Match bullets). I was using a Lee Deadlength seating die but I have the same problem with my HBCS dies as well. For the first cartidge I set the OAL so that the bullet would be about .020" off of the lands. I loaded 50 rounds and I was pretty sure everything was stable and proper set up. When I checked a few of the loaded rounds I find that now they'll be about 0.002" INTO the lands! Now I have to go back a reseat these all a little deeper. This probably explains why I occassionally felt some resistance closing the bolt in the last batch of these cartridges.

So what's going on? I almost always see some variation in OAL for everything I reload. With soft points I usually convince myself that the variability is caused by the difficulty accurately measuring the soft round lead tip.

So the variations are probably from one of 3 main sources;

1. The press. I'm using a Lyman T-Mag II

2. My ability to measure consistently with a regular dial vernier.

3. The quality and set up of the seating dies.

Obviously this will effect accuracy. Does anyone have any suggestions?
 
Are you considering the ogive of the bullet? Base of cartridge to tip of bullet overall length doesn't have much to do with whether or not the ogive will contact the leade of the rifling. There can be variation in the bullets from lot to lot. You can experiment to determine if your rifle shoots best with the bullet set into the rifling, slightly off, or with a longer jump. Keep in mind that a jammed bullet can increase pressure. The measurement from breechface to ogive contacting the rifling can be determined, and then this measurement is used to set your seater.
 
Thanks tiriaq,

Yes I very carefully measure the OAL that would put the specific bullet just touching the start of the rifling. I do this for each new box of bullets. I haven't experimented too much but I usually try for 0.015 to 0.020 length of jump. I'm very careful about not createing over pressure situations.

My concern right now is that if there are variations in OAL, they must be causing variations in case volume. If I have variation in case volume, I will have variation in pressure and velocity and therefore less accuracy........
 
Ok, you are using the best measuring system, and your jump is pretty conventional. Variations in seating depth and jump can affect accuracy. It is the bullet that is making contact? You've smoked dummies to check? How is neck tension? From a calculated jump of 15-20 thou, to bullet in contact, is quite a change. Something is odd.
 
tiriaq,

To measure OAL for contact with a given bullet I use the following method; I take a case (no primer!) and split the neck and shoulder parallel to the case length with a dremel tool. Then I seat a bullet really long and chamber this dummy round in the rifle. Then I very carefully open the bolt holding a finger on the case so the ejector does'nt rub the bullet against the side of the chamber of receiver and remove the round. Then I measure the new length of this dummy case which should be exactly the OAL which will put that bullet exactly in contact with the rifling.
 
An earlier reply mention the ogive of the bullet and you skipped right over discussing that in your responses.

Are you measuring the length by using calipers from the rear of the case, to the tip of the bullet? If you are - that is your error.... Bullets tips all vary somewhat. The bullet ogives do not.

Once you have your die set and load a bunch - why measure any of them... they will all be exactly the same if you did not change your die while loading. When I say the same I am talking about from the back of the case to a point on the ogive.... and by the same token the distance from the rifling to a point on the ogive will be the same. It does not matter where the actual tip measures.
 
Thanks guntech, that is starting to make sense. I am measuring from the base of the case to the tip of the bullet.

Not sure how to manage this otherwise. So I load my home-made tool, measure the total length (from base of case to tip of bullet) and set my seating die to produce an OAL a set distance short of that. How else can I manage this? I guess if I seat the exact same bullet that I used in my measurement and assume that the seating die is touching the bullet close enough to where the rifling will touch the ogive.....
 
Well - I believe Sinclair makes the device your talking about, but its rather pricey. Given that most reloaders are frugal, here's another option. Mark the bullet that you used to do your trial, and save it for any remaining setups for a given box of bullets, using OAL as a reference. Alternatively, seat the reference bullet in an empty unprimed case, and use the dummy cartridge for future setups, using the same box of bullets.
Sunray may be on to something... Excessive case OAL, or variation in shoulder (headspace) dimensions, may prevent the die from fully seating the bullet. Most die manufacturers recommend that you back the seating die off slightly, which would (theoretically) prevent this.
 
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