77 gr SMK seating

[rebs]

when I seat the 77 gr SMK's by case base to ogive I get variations. How much variation is acceptable and is there a way to cut down on the variation ?

 

[Kracken McGraw]

You will have to measure the bullets itself from base to ojive and either sort in groups, or find the middle ground length bullet and use that measurement for your dummy round so they won’t be too far off in either direction.

 

[dstoenner]

Since these are 77’s i doubt you are going beyond 600 so in that distance the variance of even .020 in length isn’t going to matter. Your powder charge +/- is going to have more of an effect than the oal distance changes. Just make sure your worst is no longer than mag length and you are good to go

 

[Highpower-FClass]

when I seat the 77 gr SMK's by case base to ogive I get variations. How much variation is acceptable and is there a way to cut down on the variation ?

How big are the variances you are seeing? What is the purpose of the rounds you are loading (F-Class, Service Rifle, plinking, other)?

I loaded 200 rounds of 77 SMKs yesterday. I measured the BTO for 20 of them just now, using the Hornady tool, and they all measured between 1.866 and 1.868. This is with unsorted bullets, seated on a Dillon 550, using a Dillon seating die. I was surprised how uniform they were.

What I have found to be key to uniform seating depth (be it 223, 308, 284, 6 Dasher etc.):
- Uniform seating pressure. Slam down the press handle on one, then use a slow downward motion on the next and there will be seating depth differences.
- Uniform neck tension. If there is a variance in neck tension of your cases, the bullets tend to seat deeper in the cases with less tension, even with uniform seating pressure.

 

[Longtrain]

IMO the 69-77 grain Sierra’s and the 68-75 grain Hornady and the like were developed for XTC competition, primarily for the 200 & 300 yard stages which included rapid fire feeding from a magazine. That being said, you probably can load them to mag length and have satisfactory performance. However, they certainly can be loaded longer and may give you what you’re looking for on the target.
Sierra’s are known for variations in the LTO and sorting them is the only way to skin this cat. But it’s a real PITA, and the variation load to load probably won’t show up on paper. My advise would be to find a powder charge that it likes and load them to mag length.
Now if you’re working with a Berger 77 or 80.5, you’ll probably find much more uniformity bullet to bullet.

 

[RegionRat]

In Service Rifle chambers, the typical jump or free bore dimensions mean that length variations are not that important. It isn’t difficult to manage magazine length rounds to less than 0.003” CBTO, and dimensions on the order of 0.010” probably don’t show up at the 300 yard line either.

In F-TR, the story can be very different. For example, if you chase lands versus if you have a long jump. The length variations would be dependent on how tight your jump window runs. Folks who seat up near the lands probably care a whole lot more than folks who jump 0.040” for example.

Seating stems touch surfaces that can be far from where the ogive gage is touching. Sometimes the bullet base to ogive dimension will be better on statistics than where the seating stem is riding.

More likely you will have to focus on two areas, components and techniques.

The bullets can be measured as a component up front. See if the seating stem can be used as a gage and if it is possible to measure the difference between the lengths based on the ogive gage versus the seating stem. A couple thousands can exist with the SMK and it isn’t significant or anything to worry about by itself.

Another place to look is your technique. The neck diameter, work hardening, and friction, can all lead to real seating depth variations when the stiffness of the seating set-up is low. If the neck sizes and work hardness are allowed to go out of control, then expect seating force and depth issues. Friction can also vary due to cleaning and lube technique. Friction variations have a direct affect on seating forces.

If you have access to a seating press with a force gage, you can start to isolate the contributions of each part. On the other hand, maybe for the type of shooting you are doing, it doesn’t matter???