I have been loaned a USA made Weatherby in 375 H&H. Currently I am working up loads and familirizing myself with the rifle in anticipation of using it on a hunt later this year.

The problem I am running into is that the rifle misfires about 5% to 10% of the time.

The rifle is only a year of two old. The owner fired half a dozed factory loads through it with no problem. I have fired about 130 rounds through it and have experienced about 10 misfires.

The firing pin's fall sounds weak, but I am used to Mausers so most every rifle's firing pin sounds weak. But this Weatherby sounds really weak and the primers are barely dented.

After each misfire I recock the rifle and attempt to fire several more times, but I have never been successful in getting any of the misfired rounds to fire.

To check on the primers I disassembled the rounds(leaving the primer in place) and dropped the case into my FN 375. Each of the primers that misfired in the Weatherby have fired on first try in my FN(which has a much more robust sounding firing pin fall, BTW).

Does anyone have an idea of where I should start looking? I removed the firing pin, and it didn't look dirty or over lubed.

The owner is supposed to take the rifle to back to his home country in three weeks(were it will stay until I head over for my hunt), so I am kind of pressed for time. Any ideas?

Seems strange the owner did not have misfires with his factory rounds although as you say he only fired half a dozen of such. Ten misfires from 130 odd rounds is a high average of 1 every 13.
A couple of points, is the bolt fully closed with your reloads as one that isn't suffers drag on the firing pin as the bolt is closed into battery by the falling pin?
Head space should not be problem as you are dealing with a belted case although it is possible certain brands of case have slightly narrower belts that give more head space. Set up your dies to head space on the shoulder giving a very slight stiffness to closing the bolt. The fact that your misfires will not even fire when tried again but will fire in your FN suggests the pin protrusion is a little short.

Other than your primers being a little harder than factory the only other probable cause is a weak spring which would be surprising considering the young age of the Weatherby.

The firing pin screws in and out of the bolt shroud in order to adjust the protrusion which should be somewhere at .065 - .070. A small steel ball sits in a hole in the shrouds thread and keeps the firing pin from turning when assembled. In order to remove the shroud, the cocking piece is withdrawn and rotated in it's slot until the notch on the cocking piece aligns with a shelf on the side of the notch.

As you begin to unscrew the assembly, keep your eye on the cocking notch for the steel ball which can easily fall out and roll to some dark corner. Once apart you can remove the ball which allows you to turn the firing pin in and out.

Screwing the firing pin in and out not only allows you to set the protrusion, but also allows you to increases the spring tension. You'll have to play with it some to see how it is accomplished.

The firing pin is machined with a "V" to act as a seat for the ball. As you screw the firing pin in and out, watch the little hole for the "V" in the firing pin to align and then insert the ball to hold the setting. Again, it will take some playing with it to see how it works. Be careful screwing the thing back together as that steel ball will easily fall out when you think you have it made.

I replaced a firing pin spring on a 70's Weatherby and got it put back together with not enough protrusion. 5 minutes to fix and back shooting. I say you need one turn on the pin. good luck.

It looks like the pin has about .070 protrusion. I think the problem is a combination of worn priming tool(guess the make...), and CCI primers.

I will shoot this weekend and report back.

I fixed the problem, but I still really don't understand the MK V firing pin adjustment....

Well, long story short, measured the firing pin protrusion and came up with .069". Because this seemed about right I did not try adjusting the firing pin by removing the ball and turning it in and out.

As a last resort(at 2am) before I returned the rifle to its owner I decided to try adjusting the firing pin even though I did not want to affect the protrusion.

After turning the pin counter-clockwise I reassembled and measured the firing pin protrusion. It appeared to have remained unchanged(.069").

I inserted a primed case(no powder or bullet) and tried to fire it. It did not fire. I inspected the primer and found that the dent was even lighter than before.

So I cranked the firing pin clockwise one turn(two groves?) past the original position. Even before reassembling the bolt I could tell that there was a major change as there seemed to be a lot more tension on the spring.

The final result was that the firing pin's fall sounded like it now had some real punch. It is not quite "Mauser like" but it really dents the primers now.

And the firing pin protrusion remained unchanged. The cocking piece does sit deeper in the notch in the back of the bolt which leads me to believe that the firing pin is more compressed than before.