Indianapolis Ordnance Sten Issue

[tj214]

I have an Indianapolis Ordnance Sten Mk. V conversion and several friends have IO conversions for the Mks. II and III. We all have the same problem, which I hope has been addressed and solved (have already checked this site and with IO who, while very polite, state this is "abnormal").

The IO conversion uses two small-diameter springs, one for recoil and one for the firing pin striker. They give suggested starting lengths for each, noting each gun will require adjustment.

The problem each of us faces is that with the recommended spring starting lengths, and even cut shorter in 1 coil increments, the IO bolt is literally impossible to pull back (the Hulk excepted, I suppose). When the springs get too short, either the bolt will no longer fully close or the firing pin does not receive enough energy from the hammer to fire the cartridge.

Between us, we've gone through about a dozen springs with no resolution.

Has anybody experienced this, and has anybody successfully resolved the issue?

TIA.

[Lee Enfield]

Sorry I am not familiar with your firearm, however I would note that clipping coils actually increases the spring rate of a coil spring.

The Math Behind Cutting Coil Springs - EATON Detroit Spring

If possible, I would try to find another spring to use.

[breakeyp]

Dumb question but does the bolt run freely with no springs? Could something else be hanging things up?

[tj214]

Hey BE

Actually a very good question, particularly with a new-build tube. One of my buddies' Mk. III did in fact have an internal obstruction and a guy with a Mk. II had an issue with the ejector being welded just a little off center. Honing the Mk. III tube and a little judicious filing on the Mk. II ejector fixed those problems. Now all our IO bolts slide quite smoothly inside the tubes, so I'm confident that's not the problem.

Although now that you mention it, it does seem like the design of the IO bolt (the same bolt for all Sten versions) is considerably more sensitive to magazine feed lip positioning than the oirginal bolts in FA versions. I don't own any FA guns, but I see guys at the ranges and have had them test this issue. Again, this may be in isolated issue (or operator error!), but all of us have noticed it.

Anyway, good question!

[tj214]

Hey LE

Fascinating article. The interesting thing is that when fully compressed (bolt handle in the safety slot), more than 1/3 of the coils in the hammer spring are in direct contact with the interior of the guide hole in the bolt, and more than 25% of the recoil spring are as well. Per the referenced article "Any part of the coil which makes contact with anything becomes inactive, that is it no longer functions as part of the spring." This presumably means those coils are lost to bolt return & firing pin hammer strike strength (I'm an EE, not a ME). Unfortunately this knowledge doesn't seem to solve our problem.

The recoil spring is much stiffer than the hammer spring. Since the hammer spring is problematic even at IO's recommended starting strength (lots of primer strikes that don't fire the cartridge), I don't think I'll try substituting a "softer" spring. However, I will try different recoil springs and see how they work.

Thanks for your suggestions.

[tj214]

Unexpected solution that's working so far. The Indianapolis Ordnance mod is designed to use two springs, a stiffer recoil spring and a hammer spring. We completely removed the very stiff recoil spring from our Stens and are successfully operating with only the hammer spring cut to the recommended length. Each of us have fired 3-5 full mags in this configuration with no jams or other problems. Can't guarantee this will work for everybody, but it's a very simple check if you have the same problem.

[browningautorifle]

That's the problem with taking a successful design and radically changing it to operate differently. We don't know what we're getting into. Glad it's working out though, I know the feeling of having something that won't work right...

[Peter Laidler]

The theory and practice of springs in relation to SMG's is quite complicated and strange. At a point it's one of the areas where at a certain frequency, something strange happens. I will do a bit of reading and revising and do a simplified write-up. Probably not relevant here (but it might be....).

[Bruce_in_Oz]

It might be related to "harmonics" in the spring.

Anyone familiar with "spring reverbs" in guitar amplifiers will get the picture quickly.

Here's how it was explained to me:

In something like a Sten, the "return spring", in launching the bolt forward, "uncompresses" to provide the energy to propel the bolt. Like a tubular chime, it will have a fundamental "frequency", closely related to its overall length, and several harmonic frequencies. All of these will be triggered, so to speak, when the sear suddenly releases the bolt. In the time that it takes for the bolt to run forward, strip a round from the magazine and chamber it, there will have been several of these "harmonic" waves traveling up and down the spring. If a "compression front" of one of these waves arrives at the bolt at the instant of cartridge pickup, it will reinforce the forward motion / energy.
If the harmonic wave has reached the bolt and started to reflect, it will subtract from the deliverable energy at that moment.

Not complete cancellation, but essentially partial wave cancellation.

Given that second, (even / "nice" harmonic) and third ("ugly") harmonic and so on, are happening, maybe the best idea is to have a return spring somewhat more "butch" than theoretically required.

And that is just for "single shot" from an open bolt. It probably gets more interesting when the switch goes to "FUN".

"Spring-bind" is a bad thing all round in either case.

[Brit plumber]

Trivial bit of info but I find interesting non the less, the Mauser Bk27 cannon, when operating at its natural speed, the return spring never actually extends, the gun cycles quicker than the springs can extend. Only on the last shot can the springs fully return to rest. The gun is actually electrically delayed to slow the rate of fire it’s that quick. Oh and barrels barely last 2000 rounds before they are smooth bored by ware.