The hardest primer to ignite in small arms is the Cal 50 BMG round and requires lots of striker energy.
The second hardest primer to ignite in small arms is the small rifle primers loaded in MILSPEC ammo.
The large rifle and standard small rifle are the mid range.
Then comes the magnum pistol primers.
Finally the small pistol primer is the easiest to ignite and have to be because the majority of handgun designs don't lend themselves to having heavy striker energy especially in revolvers.
556MM primers were redesigned when the first M16s were going off when the bolt was released and the free floating strikers continued forward and impacted the primer.
Thus several things were changed in the ammo and the weapon design.
The first strikers on ARs had a big knob on the back where they have a small knob now. Obviously the weight of the striker was reduced significantly to lighten the impact on the primers. In the business such ignitions are known as TIC OFFs.
Also to stop TIC OFFs the primers were redesigned to have heavier cups requiring more striker energy to set them off reliably.
The M16 spec calls for a .022" indent on copper for reliable ignition. Coppers are very expensive and there may have been another way to measure it developed since I retired.
Ruger/Remington at one time wanted .020" indent on rifles. Other vendors wanted only .016" indent which last I read is the SAAMI min recommendation now.
You have to have a holder for each caliber you want to test.
Above are the holders for 30.06, 7.62 and 5.56MM.
Above is a copper inserted into a copper holder and once inserted the weapon is held muzzle down and trigger is pulled resulting in striker impact the copper.
This is a bench inspection gage to determine how deep the indent is on copper pellet. Note gage in this photo is zeroed.
After impacting the copper it is put back under the gage and the tip of the gage is inserted into indent. In this the indent is showing just shy of a .019" indent.
To my knowledge coppers are only made by Winchester now and they are pricey. The holders are made by Pacific Tool and Gage and last ones I bought years ago were 155.00. I also have a holder that fits most of the magnum rifle calibers. I heard they were $1.00 each about ten years back and they are precision made.
Note the indent on the copper has no relation to the indent you get on a fired case and deep indent on fired primers means only two things, the round went off and your striker is not broken.
Now on some rifles with insufficient energy you will see evidence of reverse primer flow that tends to let the primer surface to push the striker back and follow it into the bolt face. To be sure the round will fire but if the flow is bad enough you will see a hole where your striker hit, looks like it was drilled. If this happens pull your bolt down as chances are very good the piece that broke off is now inside your bolt and it stops the striker from protruding correctly to insure ignition.
CIP I bought a new Remington 7615 223 Police Patrol Rifle about 18 years back. It had a crooked bore and I could not sight it in at 100 yards and showed reverse primer flow. I sent it back to Remington who replaced barrel. Shot fine when it returned and still had reverse flow. I ordered new hammer spring from Wolfe, put it in and the reverse flow disappeared immediately.
Bottom line you only want to reload 223/5.56 with MILSPEC Primers. Last I heard Remington BR has heavy cups, the Russian primers do and CCI has a primer with box marked milspec.
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