A very good friend has given me a 1908 Mannlicher. I have tried without success to find loaded ammunition in Canada but no luck.

An amigo ordered and delivered to me a set of dies and 40 brass. This is 35 Whelen brass that has been reformed to 8x56 MS...

I loaded up a few rounds with 180 grain Barnes ... 43 grains of 4064.

I went to the range, shell went in smoothly and fired one shot at the twenty five yard range ... The rifle is spot on but the primer was protruding ... Nuts ... No other signs of pressure ..

I did a bit of research and was informed that I was underloading the shell ... Hmmm...

So I reloaded 44 grains and 45 grains and tried again. Cases are now separating with flattened primers .. One brass was a total separation ..

And to top it off ... I noticed with the help of a friend who pointed it out to me that the shell will not fit into the rotary magazine .. Too big ... what in hell is the problem ???

I am going to get some caliper to measure the fire formed case to see if I can figure things out myself but ....

No calibre is on the outside of the fine little rifle ... the previous owner had never fired it for lack of ammo ...

Help ...

How about a chamber cast to find out exactly what you have?

For sure your brass is too short for that chamber, headspace wise. That is the easy part. Fire form that brass for the chamber and don't set the shoulder back. I fire form by seating the bullet out so far that it jams into the rifling, hard. With a medium load. Do not fire it until you get that sorted out. Under loading is not your problem.
As for the magazine, MSs basically won't feed with anything other than what they were made for; someone who knows them should look at it.

Thank you so much ..

Just realized you were using reformed 35 Whelen brass; it is larger than the 8x56 and will never fit into the rotor. Even a few thousandths is critical in the MS magazines.

I used new Winchester 8x57 brass and an 8x56 MS full length die and resized the cases until they would barely allow the bolt to close. Then trim length to 56 MM and reload - shoot.
The brass should resize all the way down the case and should function in the magazine.

A chamber cast would be a good idea.

Did any of your rounds fit and function thru the rotary magazine?,,or did they all fail to seat into the rotor.

If the latter,,you may have a mag assembly (bottom plate and rotor) from a 1903 in there.
The cartridge cut-outs on the rotor are smaller in dia for the 1903's 6.5x54 case (as well as the obvious other differences).
The 8mm will sit up high on the edges of the rotors cut-out for the smaller dia 6.5 case.

An original magazine assembly/rotor should be matching ser# to the rifle.

As far as the primers backing out,,do as dpcd says and fire form the brass to the chamber to get the headspace right. These are noted for being less than perfect for headspace so you can't trust the reloading die to form a case that perfectly matches your chamber (not that you ever should).

Also the rifle may have been altered at some point for standard 8x57 Mauser. That'd ever so slightly lengthen the chamber and headspace over the original 8x56 but more than enough to give you the set back primer condition you show..

8x56M/S is a few .000 small in dia at the head than standard 30-06 based cartridges. The rim is the same.
Forming the 8mm from any of them may require that they be reduced in dia those few .00 at the base for chambering and even for fitting the cartridge cut-outs in the spool at times.
Some will take them with no alteration, some will not.
The 1905 and 1910 are the same issue.

My rifle is at a gunsmith right now. The chamber cast shows it to be the 8x56 as I had assumed. He will further assess it but my kindly amigo's gift is not without problems ...