How do I determine best OAL when reloading?

14 August 2002, 08:58

<green 788>

I would seat a caliber's depth into the case, and then begin with five or six variations of powder charges.

When the powder ignites and the bullet begins to move, it obturates, which means it bulges like a soccer ball being kicked. If the bullet is so close to the lands that it actually engages the lands while in this obturated state, it can deform to a degree. The deeper seating helps the transition from case mouth to throat occur less tumultuously.

In a match grade rifle chamber, there won't be as much room for the obturating bullet to spread itself into, and on such barrels, shallow seating often works. It will even work on factory barrels if pressures are kept low enough.

But you're shooting a 22-250, and you want velocity.

I'd go with a 55 grain bullet, seat it .22" or so into the case, and use the following powder charges, loading four of each variation:

IMR 4895 (purple can)

33.4 grains
33.7 grains
34.0 grains
34.3 grains
34.6 grains (Watch for pressure signs along the way).

Shoot at five different targets, one assigned to each powder charge being tested. Shoot one 33.4 grain charge at target one, then shoot one 33.7 grain charge at target two, and continue in a "round robin" fashion until you have five three shot groups on paper. (You'll have one round of each recipe left over, which I'll mention later).

By alternating targets this way, you won't disadvantage late groups due to a fouled bore or heated barrel. Do allow time to cool, however, and it might be wise to clean half way through and shoot a couple of foulers before continuing.

After you have your targets, you're not looking for the tightest group of the five. That could easily be a fluke. You're looking for the three groups which come the *closest* to hitting the target in the same spot.
You then choose as your load the powder charge from the center of this trio.

Why do you want this? Because you want a load that can tolerate inconsistencies in brass cases, powder lots, outside temperatures, etc. The .3 grain difference in the powder charges will simulate pressure rises and drops caused by several things.

Have you ever had a really tight shooting rifle that liked to throw a flyer every now and then? While there are many causes for flyers, one cause is a pressure change in the load, caused perhaps by an odd brass case or primer variation or the like.

I call this method of load development the Optimal Charge Weight, or OCW load development method. It has worked well for every rifle I've tried it on.

When you have selected the powder charge from the center group of the three, take an additional three shots, this time all on the same target, consisting of the low, median, and high charged load. If all has gone well, you'll note that these will all three fall into a pretty tight group in spite of the .6 grain powder charge variation.

I would predict that 34.0 grains of the IMR 4895 with the 55 grain bullets will be the OCW for your rifle, as it was for mine and one other rifle we tested. You can probably skip this whole test and go right to 34.0 grains of IMR 4895 and vary seating depths and find a great load that way.

H380 is another fantastic powder for the 22-250, and you'll find a lot of good information here regarding the use of that powder in this application. If you want to use my load development method with 55 grain bullets and H380, I'd go with 35.2, 35.5, 35.8, 36.1, and 36.4, watching carefully for pressure signs along the way.

As a *final* step, you would alter seating depth to tune the group to ultimate tightness.

In my opinion, beginning with a pre-conceived OAL and altering powder charges until you get tight groups isn't the best way to go, because you may well arrive at a powder charge that will be very sensitive to cases, powder lots, temperatures, etc. The best amount of powder to have in a particular recipe is the amount which ignites and burns most consistently. For every load recipe, one powder charge (give or take about 1%) will meet this requirement. Find out what that charge is by the above method, and fine tune with OAL adjustments, and you'll have a resilient, accurate load...

Dan Newberry
green 788

15 August 2002, 01:24

Hot Core

quote:
Originally posted by Shark Bait:
My question is where should I start testing off of the rifling? It has been suggested that I start at .020 off. Should I start with the bullet touching the rifling and back off in .005 increments? Should I start at .020 and go in at .005 increments?

Hey Shark Bait, It does seem that a good many "rifles" shoot their best with the bullet's Ogive close to the Lands. That is how that old Rule of Thumb got started.

But, due to some bullet's shorter lengths, higher Ballistic Coefficients(sleek Ogives), and some rifle's long throat lengths, the combination of those bullets and a specific rifle does not always allow "Seating - close to the Lands". In order to shoot those bullets, many folks find being off the Lands "farther than normal" is not "always" as big of a handicap as first imagined. It just depends on if it is possible to locate a Barrel Harmonic where the Case/Primer/Powder/Bullet/Rifle combination shoots well.

I like to begin my initial Test Loads with a new bullet seated to Kiss-the-Lands when possible. Then I develop the load increasing the Powder until I see Pressure Signs that tell me to STOP adding Powder.

The next series of Test Loads "might" be done as Dan suggested, looking for the load that will group on the Harmonic and on each side of it. Then go back and fine tune the Seating Depth(as you originally mentioned). Use either 0.005" or 0.010" Seating Depth changes as you desire. I generally run the second set of loads with 0.010" Seating Depth variation and the third set with 0.005".

But, there is really nothing wrong with starting 0.020" Off-the-Lands and Developing the Load from there either. Plenty of ways to go about doing this and they will all work. That is why you see various people recommending different methods.

I'm not in near as big of a hurry to get a Final Load(Hunting, Target, Plinking, etc.) as some folks are, so shooting a few more bullets to get to the Final Load is not a big deal. Plus, it gives me more Trigger Time and Pressure Data which increases my confidence in the Final Load.

quote:
Originally posted by green 788:
I would predict that 34.0 grains of the IMR 4895 with the 55 grain bullets will be the OCW for your rifle, as it was for mine and one other rifle we tested. You can probably skip this whole test and go right to 34.0 grains of IMR 4895 and vary seating depths and find a great load that way.
Dan

I suspect Dan is not aware that 34.0gr of IMR-4895 when used with a 55gr bullet is shown as a MAX Load in Hodgdon #26. The same load is not shown as MAX in some other Manuals however.

So, I'd recommend you ALWAYS "start low and work upward "IF" the Pressure Signs allow it". This extremely old Reloading Rule is as Valid today as it has always been.

You might not be able to reach 34.0gr of IMR-4895 SAFELY, you might be able to reach it SAFELY, or you might be able to exceed it SAFELY. But one thing is for sure, you will never know if you are on the peak of the Harmonic if you choose to just try the one Load(which may be UN-SAFE in your rifle).