MISFIRES! ...String passing underneath bolt!

Hi Everyone. I have found my way back here again as I have started doing a bit more crossbow of late.  Firstly I am a longbow-maker and as such am totally bow and archery savvy (though by no means do I suggest I know it all!).  Crossbow is a bit a side-line, but an interesting side of archery I am exploring ...and at an early stage really. I am however getting a persistent problem with crossbows and bolts that I make or restore.

I have made an early medieval wooden crossbow (posted a while back on here), and am currently restoring a 'classic' rifle-stock sporting crossbow which is a complete individual creation of someone unknown, which I found in a market/emporium. It is an alloy prodded target weapon (nothing whatever to do with Wham-O Powermasters!) which I can tell has been very well designed and made, both from a mechanical and bow-design viewpoint. I have made a string for it and leather-backed the alloy prod for safety and generally cleaned it up etc., but when I test-shot it twice I had the same kind of misfire where the string goes under the bolt.  This bow has a deep bolt groove for a 3 fletch bolt and I made a few test bolts from old field arrows by cutting off the nock basically and putting on a pile at the right length.  I had this problem at first with my medieval bow also - though this has a shallow bolt groove and uses two-fletch bolts, and the problem seemed to disappear (apart from the odd one every now and again) in general adjustments etc. 

I wonder does anyone know why this might be happening?  Can the cock-feather of the bolt lift a bolt up if it even slightly compressed in the groove? It creates a terrible dry-fire which I want to avoid as it rips off the cock-feather of the bolt and risks the bow and the string , and there might be a simple solution I know nothing about  .  I say again I am new to crossbows - making, repairing, setting-up etc. and although I will be able to understand all the practicalities to their operation, I know next to nothing and am at the bottom of the learning curve!  Any suggestions will be very happily received 

what kind of lock is it?

Yep -- need to know the kind of lock.  Close up pix would be nice...

Hi guys. The lock is a developed form of medieval rolling nut. it is 'developed' by having an extra spur on the nut that resets it when you span for the next shot. It rolls the nut back when you pull the string past the it. The lock is essentially though, the traditional medieval type

Questions to ask in case of string-under misfire.  Are the string and bolt approximately the same diameter?  If you have 3 feathers and cock feather is down, is the bottom feather lifting the bolt on release?  Are the bolts truly square on the back (slightly concave across the back is okay) Do you have a bolt-clip that is interfering with release (I once had a bow that misfire only if the bolt-clip was in use, but not if it was turned around, free of the bolt.
Try this: set the lock, without a bolt, put forward pressure on the roller with your finger, now slowly release the trigger.  Are you getting odd movement in your roller (does it roll back as you're releasing? Does the roller actually rise in the socket during release?) Does your bowstring in the roller's lugs actually lie in line with the bolt's butt, or is the it sitting too high?  Does your reset-lug somehow interfere with to roller's free movement?  Fact is, if your string starts in contact with the middle of the bolt's butt and nothing disturbs it during release, there's no way your bow should misfire.    Keep testing... Geezer.

Justa thought, but maybe the roller nut's two jaws are too curved backwards, and they pull the string downwards on release. I'm probs wrong, but you never know.

My very first medieval crossbow misfired consistently when I used the rather stiff bolt clip. If I removed the bolt clip it shot allright.  Eventually I discovered the very heavy brass roller was letting the string ramp up the backs of the lugs on release.  However, with the bolt solidly engaged with the accelerating string, the bow would fire okay without the clip.  With the clip engaged, the bolt got stripped off the string and dropped on the table.  In your case, with the string going under the bolt, there's always a possibility your groove is too shallow and the string too narrow and too low in the lock. Since you have a deep groove to clear your cock-feather, I suggest you widen the upper portion of the groove and use a thicker string.  That will probably 'fix' the problem functionally, but there may some deeper problems with bow/stock/lock geometry that you will eventually discover.  You shouldn't have more than a 1 percent misfire rate, or something is wrong. Beware, bolts that have consistently misfired or near misfired may get eroded butts that allow the string to slip over or under.  If they're clearly un-square, fix that.   Good luck and keep us posted.  Geezer.

And yes, before  you ask: it is possible to glue old auto-pistol brass, say 7.65 or 9 mm to butts of bolts to make them impervious to damage, but that WILL add weight to the back of the bolt... which is precisely where you don't want it. Center of balance should be 1/4 to 1/3 of way back from the head.  Anything further back tends toward instability in flight.   In the old days guys would load a flat-nosed blunt in a crossbow and shoot upwards at a high angle... the bolt will turn around in mid-flight, to the delight of onlookers (yeah, people were pretty bored in those days and hence relatively easy to delight) Yes, I have tried, and it works.  Geezer.

Thanks for your feedback guys. I think I might make a new string. The one I made is a Fleming twist and it was hard to make it properly round in section in such a short length. Although I don't really like them, I think I will make a running-line string as I will be able to be sure it is completely round in section at the lock. Make sure it is thick enough too

Right guys ...Another spectacular, glorious misfire/dry-fire after a new string and slightly concaving the bolt nock and checking out the lock (as suggested). After your advice I think I know what is wrong now ...a design fault. The flight-rail only has a groove for the cock-feather ...there is no recess for the bolt to lie in.  It can never be aligned with the string and the string will always be only just contacting the bottom edge of the nock. There is quite and angle on the string across the rail down to the string nocks. I only saw the crossbow unstrung ...so coupled with my lack of crossbow-specific experience and knowledge I did not pick this up at the time.  I was only struck with the good look of the weapon and the very good design of the prod ...which also has a superb tiller I can see, now I have strung and spanned it. I do not know what kind of bolts the maker used with it. I guess my next move would be to change out the flight rail liner and replace with one with a slight recess to take the shaft of the bolt as well as the cock-feather.  That way the bolt could sit lower and be aligned more centrally with the string.  I attach a few pics anyway  ...or I would if I knew how (don't seem to be able to get the hang of the 'hosting' function ...despite repeated reading of your instructions and attempts no url appearing ...??)

You've got it!  Deepen your bolt groove, so center of bolt's butt lies in line with center of string.  That should solve your misfire problems. Happy shooting.
Geezer

Thanks Geezer! (...I would love to send some pics, but am mystified by the 'hosting' method of sending from my computer despite trying it repeatedly and re-reading instructions on this site. No URLs appear after I send pics. Maybe I am missing something? I am a bit of a luddite ...but not a no-hoper when it comes to IT and computers! I must be missing something obvious!!  )

Got it! ...Just being impatient!  This shows the bolt against the string (upside down!)

Here's a pic of the complete crossbow - in my messy workshop ...I was certainly impressed by the look of it. What do you guys think? (I know the brace height is a bit low ...due to dry-fires!). It looked worth restoring to shooting condition to me

Simple solution is to use 2-fletch bolts, or 4-fletch with the bolt resting on two 'legs' of the X not on a single feather.

I wonder if it was meant for capture nocks like a vertical bow?  Makes the bolt clip somewhat redundant maybe...

In any case you could file a bolt groove with a square file held at 45 degrees until your bolt sits at the right depth.

Interesting story... thanks for sharing