Moving the nut lugs forward.

Ive read somewhere that moving the lugs/ fingers forward of center on the roller nut helps reduce missfires.
1. Is this true?

2. If so then by how much?

Fester,
I'm wondering if you're asking because you are experiencing misfires or not. I'm thinking hard and I don't remember ever having a misfire with a roller nut lock mechanism on any of my builds. I make all my roller nuts to the same basic geometry, with the sear for the trigger lock and the center of the string as exactly opposite of one another through the center of rotation as possible. So the lugs are slightly forward of center, but only to accommodate the diameter of the string. Does that make sense?
I know that adjusting that geometry will impact the amount of pressure required to release the nut, but I am not aware of any impact on how the string interacts with the bolt. In my experience that has always been a matter of the string's position relative to the bolt track and being centered on the body of the bolt.
Gnome

Moving the claws off-axis has a side effect of making them shorter in height; to keep them as tall as possible I like to have the tips of the claws centered with the axle. The only misfires I've had so far was when the string 'climbed the claws' on one build due to short claws and bad geometry.

The last nut I made has the sear placed behind the axis by about 2-3mm, similar geometrically to moving the claws forward...the result was a very secure lock, and a pretty stiff trigger pull. I was told that moving the sear the other way (in front of axis) would cause misfires.

Ah, maybe I need to get my terminology straight. I tend to think of "misfire" in firearms terms, as in you fire the weapon and something unexpected and unwanted happens. Are we talking misfire as in the lock or string slips and the weapon fires unintentionally?
That has only happened to me once with a roller nut, attempting to draw and lock my very first build for the very first time. That was a 205 pound steel prod, it held for a second then released and nearly took my fingers off. The problem turned out to be a sloppy angle on the sear and the locking end of my trigger being too rounded, not enough flat surface area to engage the sear properly. Now I always us cocking ropes, even with prods I could draw by hand. I like all my fingers and want to keep them.
Gnome

I had one or two go off unexpectedly. No fun.Since then I always make sure that my claws always have a slight cant to the rear of the lock . This way , there is no possible way the string can jump off the claws of the  nut.I use a similar setup for the self lock where the string is pushed off the edge of the stock. If you check commerical crossbows you'll see this . It just makes for a safer design.I might add here that proper design for the nut dictates that the bottom of the nut where the string would lay, the axle, and also the front edge  of the sear where the trigger would bear, should all be exactly on center. The trigger should be on the center of the sear.

Here some informations... http://crossbow.wikia.com/wiki/Designing_medieval_nut_and_trigger_crossbow_locks

I just wonder what experimented people think about that

Dark, A great find you had there. I also like going through the different links. Notice what I mentioned is in there.

the website is interesting and seems to be serious and well documented (the traditional Bowyer's Bible seem to be an important reference too, which is THE reference for bows). But I don't have enough experience in crossbow and nuts to say if all is true or not.

Dark ,it's true as you read it.  The position of the fingers in reference to the sear can make a big difference in the holding weight for instance. That amounts to how hard the trigger is to pull. It can also make the grip on the string more or less safe.

I've also seen you tube clips were the lugs are clearly slightly forward of of center.

I've been building medieval style crossbows a long time, so I have a lot of opinions on the subject of roller locks.  Take them for what they're worth:
1: your roller nut should fit closely in its socket and turn Very Easily.  Heavy rollers have more inertia: they accelerate more slowly and particularly if they're tight in the socket, that can contribute to misfires... the string climbs right out of the lugs.  Lower lugs, mounted ahead of center may have slightly more tendency to misfire.  They may also release a bit faster, which is good.  Most medieval bows I have examined, have the back edges of the lugs approximately in the center of the roller. A few, like the Maximlian Bows in Vienna, appear to have rather longer lugs, that pass behind the top-center. 
2: Payne Gallewey shows his roller nut with @ 3/5 of its diameter below the top of the stock (in a steel socket) in my experience that's too shallow.  I recommend sinking it by 2/3.  As one approaches 3/4, you can no longer remove the roller from the top of the socket... you'll have to take it out the side, which means you have to drill your roller-socket from the side, rather than using a mortised lock that goes into the top.  PG also places his lugs at top center and his iron sear at bottom center. Given a relatively shallow nut, particularly with a slightly undersized roller,  the nut may try to rise out of the socket under load, which increases wear on the edges of the socket.  The socket can be reinforced with bone or metal, which should deal with the problem, but I have discovered moving the sear-plug back about 1/4 inch reduces the load on the edges of the socket, transferring it to the trigger.  Of course moving the sear can make the trigger harder to pull, but that can be remedied by careful adjustment of the sear-face and the end of the trigger, so that the trigger end (distal end) drops straight down out of the sear, rather than forcing the nut to roll back on release.  Many bows made in late 15th century used a two-axle lock that carried the sear plug on the Back of the roller rather than the bottom.  They work fine that way.
3. Bows that tend to misfire... particularly ones in which the string climbs out of the lugs on release, going over the string (may mean you have too little down-pressure on the string or a string that's too narrow, or groove is too deep... the possibilities are numerous) Sometimes misfire can be cured by cutting a hollow in the backs of the lugs... make them concave. Some medieval bows are done that way, but certainly lots of stronger bows with very fat bowstrings just had flat surfaces on the backs of lugs.  Again, a heavy roller tends to exacerbate misfire problems.  My first crossbow had a heavy, wide brass nut.  I had endless problems with it.. eventually cured most of them, but the thing still misfires occasionally if I use a bolt clip (another set of problems there... if your bow misfires with the bolt-clip but shoots fine without, that means the string is rising out of the lugs, but carrying the bolt with it.  When there's no clip, it may shoot pretty well.  With the clip in place, the bolt is stripped off the string, which goes over the bolt, throwing it into the ground.  Endless entertainment.  Everybody has to start somewhere and inevitably everybody runs into these problems.  Persevere!  Geezer.

Great post Geezer.

Has always surprised me while old roller nut mechanisms were rarely, if ever fitted with a latch cover as per modern crossbows? Provides both a base for a rear sight, and attachment point for a clip.... and most noteworthy, prevents any chance of "misfire" (string riding over bolt)

I guess they just had their trigger geometry nutted out (excuse pun) back in the day!

Yeah, in fact, most medieval bows didn't have any kind of bolt clip.  I suspect the bolts were heavy enough, that they could be wedged between the nut-lugs and still be undisturbed as the roller turned.  With modern lightweight bows and bolts, having a tight lug/bolt fit can precipitate problems.  There are a few period illustrations that seem to show the shooter holding the bolt in the nut with a finger of the off hand (off hand right under the lock, finger slips over the bolt's butt, behind the bowstring) According to William Paterson's book, bolt clips don't show up till late: maybe 15th century.  Personally I suspect that's a bit too late... I mean, the concept is dumb easy, and the fact we can find illustrations where shooters are doing the same thing with a finger, recognizes the need.  Geezer.

And in fact, I fit all my pushpin/notch crossbows with a stiff, flat bolt clip that simultaneously holds the bolt down and prevents the string from riding over the bolt.  But in fact, I've never seen such a device on a medieval bows (okay, there are some weird things from 15th century on, particularly from Italian designers that may qualify... there are even some self-resetting locks)  Geezer

Yep, nut lug covers seem like such an easy solution for potential "string over" misfires, and keep the bow safer before the shot.

Geezer wrote:Yeah, in fact, most medieval bows didn't have any kind of bolt clip.  I suspect the bolts were heavy enough, that they could be wedged between the nut-lugs and still be undisturbed as the roller turned.  With modern lightweight bows and bolts, having a tight lug/bolt fit can precipitate problems.  There are a few period illustrations that seem to show the shooter holding the bolt in the nut with a finger of the off hand (off hand right under the lock, finger slips over the bolt's butt, behind the bowstring) According to William Paterson's book, bolt clips don't show up till late: maybe 15th century.  Personally I suspect that's a bit too late... I mean, the concept is dumb easy, and the fact we can find illustrations where shooters are doing the same thing with a finger, recognizes the need.  Geezer.

I've often wondered about this. when shooting down from the battlements of a castle wall how the bolt was prevented from just falling  off the bow. I did read in a history book "Castle. By Mark Morris" That defenders overcame the problem with a dollop of bees wax. Have you heard any other references to this Geezer?

Beeswax: makes sense to me.  I have seen the idea mooted about for notch/pushpin bows.  Geezer

I've once red that bolt clip are used after Middle Ages, not before. (Not sure the book is a good reference). But medieval bolts I've seen are cut at the notch to enter into the nut "fingers". In the book I speak, the author say the bolts were a bit blocked between the nut "fingers" so this can't move at all... not sure if that's possible.
I've also heard about resin that "glue" the bolt on crossbow... cherry tree resin could works for that.