7mm mag brass stretch

[Adirondack]

A friend bought a set of Rcbs dies to reload. I told him we would reload some together. He sent me home with 60+ pieces of mixed Federal, Hornady and Winchester brass. I tumbled them all and I used my Hornady cartridge headspace comparator and measured a good bunch of the brass. It's all within .001" of one another, regardless of brand. I used my universal decapping die to make sure the primer was not protruding out past the cartridge head.

Now, I don't own a 7RM. I had no idea how much the shoulder needed to be pushed back to set up the FL sizing die. I set the die to bump the shoulder back .002". I stopped at that point until I could measure a factory round.

He brings me a factory Hornady round today. A 154gr SST. I measured it tonight and it is a full .014" less than the fire formed rounds! This seems excessive. I have never had this problem with any non-belted cases in my Savages. Should I see if I can get ahold of another factory round or advise him to get the headspace checked? It's a Remington 700.

I dont know how much is normal. What would the difference be between a set of go-no go gauges be? Surely not .014"?

[hardnosestreetcop]

Adirondack: cartridges cases can vary considerably from manufacturer to manufacturer. The composition of the metal frequently will differ, the overall length often varies even in new cases from the same manufacturer, and certainly the internal capacity will often be different. Because of these variations you should never mix the case brands, minor differences like these can translate into variations in pressure and how the load performs. Once one brand of brass has been fired form to that rifle's chamber, after that neck sizing will aid in preventing the cases from stretching.

[Adirondack]

Adirondack: cartridges cases can vary considerably from manufacturer to manufacturer. The composition of the metal frequently will differ, the overall length often varies even in new cases from the same manufacturer, and certainly the internal capacity will often be different. Because of these variations you should never mix the case brands, minor differences like these can translate into variations in pressure and how the load performs. Once one brand of brass has been fired form to that rifle's chamber, after that neck sizing will aid in preventing the cases from stretching.

Thanks for the reply.

What shocked me is how much difference there was at the shoulder from new to once fired. Since posting, I have been doing some reading and this is a normal occurance with Belted Magnums. I will sort the brass by brand as recommended. I did the final setup on the sizing die last night and I am consistently bumping the shoulder back between .0015 and .002. I am pretty happy with the results.

I really like my Hornady cartridge headspace comparator.

[hardnosestreetcop]

consistently is the key, the more variables that can be eliminated will aid in obtaining tighter groups. Good luck

[RC20]

Also, the chamber is going to dictate what the brass does once its fired.

That indicates that the headspace is on the long side or the chamber is reamed deeper than normal.

Once its formed though, then you push back the shoulder based on the fired brass not factory new.

That assumes its out of his gun.

[hardnosestreetcop]

The way I control case stretching is I neck size em most of the time

[Robinhood]

RC20 is correct. On a belted mag you set the barrel up to the headspace gauge that locates on the rim. After that you measure a fire formed round and document the base to shoulder datum dimension. The case headspace, base to the shoulder will be specific to that rifle. .014 is not unusual from one rifle to the next. Bump your shoulder to -.001-.003 of that rifles chamber to keep from stretching the brass to much..

[strut64]

Are you sure that the brass will fit in the chamber of his rifle?? You should try your resized brass as the next step.