I recently got a Sako L46 back from the gunsmith who I had rebarrel it with a .300 Blackout barrel. It's putting my handloads with 130 grain spitzers (2,170 fps) into delightful half-inch groups at 100 yards.

I tried a number of combinations of handloads using two different powders, three different bullets, and two different primers (CCI 400 and Sellier & Bellot SR) before settling on this load. All fired perfectly.

After finding a great load, I decided to try some of the Hornady Subsonic 208 grain ammunition the gunsmith had sent along with the rifle. He had fired two, and left the remainder of the box for me. It was very frustrating shooting that ammunition because it mostly failed to fire on the first try. Sometimes it would fire on the second attempt, sometimes on the third, and sometimes not at all. I think that of the 18 rounds I attempted to fire, about 5 fired on the first firing pin fall, 5 on the second, 5 on the third, and 3 never fired and were discarded.

Now, I checked everything: The firing pin dents on the unfired Hornady ammunition were fairly deep and normal looking. The firing pin protrusion from the bolt was normal and within specs. The firing pin spring seemed as strong as similar actions I own. AND, in firing over 100 rounds of handloads (some in new cases, some in FL-sized cases, and some in neck-sized cases) with two different makes of primers, there was not a single failure to fire or hang fire. AND, firing five rounds from a box of Federal American Eagle 220 grain subsonic also yielded no firing failures. So, three different primers all fired normally, while the primers on the Hornady factory ammo did not.

Headspace: If you're wondering, headspace seems perfectly normal with all ammunition tried. Many of the handloaded rounds were assembled in newly-purchased Hornady brass, which was presumably identical to the Hornady brass of the Hornady factory ammunition. There was no apparent movement of the shoulder when the Hornady factory ammo did actually fire. Besides, four different sources of brass were used: That of the Hornady ammunition, purchased Hornady new brass, remanufactured military .223, and that of the Federal factory ammunition. All fit and fired perfectly. So, headspace isn't the problem.

Bottom line: I can only conclude that Hornady used a bad batch of primers on this lot of ammunition. The fact that many of them fired on subsequent firings indicates that it was not a problem with the powder. The fact that firing pin indentations were normal looking (and three other primer brands fired normally) indicates it is not a problem with the firing pin/spring.

By the way, the velocity variations of the Hornady ammunition (when it did fire) ran from 946 fps to 1062 fps (21" barrel). That is a huge percentage spread, which seems to be an indication that when the primers did fire that there may have been erratic ignition of the primers.

Now, to assure that this is not simply an anti-Hornady rant, the very best accuracy obtained (less than one-half MOA) was with Hornady bullets.

Your Bottom line seems to nail it.
I had a similar problem with a .257 Roberts Ruger circa 1970. A fair number of times it failed to fire when using CCI # 34 primers ( anti slam fire primers ) ALL other primers worked great. roger

I have read that Hornady is using #41 primers in their 6.8 SPC cartridges. I wonder if Hornady is using the No.41 NATO-Spec Small Rifle Primers in their 300 Blackout ammo. The military primers have stronger cups designed to reduce the chance of a slam-fire in ARs. Velocity differences might also result from some primers being hit hard enough to function properly and others being hit barely enough to set them off.

I would say from the looks of the firing pin dents on the unfired rounds that the Hornady primers were being struck hard enough (regardless of how hard the cups might be). But it is possible that the primers might be more consistent if struck harder (as indicated by those which fired on the second or third try).

The CCI 400's which I used in most of my handloads have a reputation of being on the hard side. In fact, I own an Oregon Kimber 84 in .223 which sometimes misfires with the CCI 400's, so I load Win WSR's in ammunition for it. However, the CCI's performed flawlessly in this rifle, which seems to be another indication that the Hornady primers are defective rather than just too hard (or possibly so hard that they might be considered defective.)

Since I used up the entire box (purposefully, not wanting any ammunition prone to misfire laying around) it is impossible to test it in another rifle. But since the Sako seems to have a pretty firm firing pin fall I tend to doubt that another rifle would have acted differently -- however, I have no idea how the energy of the strike of a Sako firing pin compares to that of the AR-style rifles most commonly encountered in this caliber. So, I remain open to the theory that the Hornady ammunition is purposefully primed with very hard primers. Wonder what kind of primer strike force the recent Ruger American exhibits in this caliber?

Forgot to mention: This rifle was formerly a .222 with an aftermarket barrel of unknown origin, which is why I was willing to rebarrel it (I would never defile a classic Sako which had a good factory barrel on it.)

Anyway, while I only shot it a little before rebarreling, it never suffered any misfires in its incarnation as a .222. More evidence that the problem is with the ammunition and not the gun.