[macavada]

I've been lurking around this and other forums for a long while now, and I still have not seen where someone has explained two terms that get thrown about quite a bit. Is there a link to a sight where terms such as trigger creep and headspace are defined? Or, can someone please explain these to me? Really, I'd like to find a glossary somewhere or some site that explains the terms that get thrown aroung when referring to guns or shooting. I've learned a lot in a short time, but I still can't figure out the accurate definitions of these two terms based on the contexts I've heard them in. How does one identify a creepy trigger? How can I tell if my Sprinfield Armory .38 super and Para Ordnance P12-45 have creepy triggers? Any responses are really and truly appreciated. Thanks.

[Jim Watson]

Trigger creep is any discernable movement of the trigger (of a single action firearm) before the hammer falls. Do not confuse the easy movement of the 1911 trigger before it starts applying force to the sear with creep. That is "takeup" and some is needed for operation of the action. But when you have moved the trigger back that distance and are really squeezing the trigger to make the shot, you should not feel it move. The old term was a "crisp" trigger. Or a "glass rod trigger." It ought to go as suddenly as a thin glass rod breaking. On a well adjusted premium gun, that is. Better some creep than risk a hair trigger on a production line pistol. Your guns probably have some creep and are safer for it. Try the trigger SA on a Smith & Wesson revolver. There is a crisp trigger, if the assembler got it right.

Some high quality European guns have "soft" triggers. There is noticeable movement, but it is so smooth and light as to be no trouble. Some shooters prefer them.

The trigger should not keep moving back AFTER the hammer falls, either. That is "backlash" and is very damaging to accurate shooting. Obviously there has to be a few thousandths of movement if the sear is to clear the half cock notch - or the modern perversion, the safety stop, but you should not feel anything. That is what the little screw in the lower edge of the 1911 aftermarket triggers is for. But don't screw it down too far, it can keep the gun from firing at all, or cause damage to the sear if it strikes the edge of the half cock in passing.


"Headspace" is technically the space between the head of the cartridge and the breechface. But it is measured from the breechface to whatever surface stops the cartridge from going any further into the chamber.

Headspace on .45 ACP is controlled by the case mouth against the front edge of the chamber, so the cartridge headspace is the distance from the head of the round to the casemouth. Chamber headspace is the distance from the breechface to the front end of the chamber.

Bottleneck rifle headspace is measured from the bolt face to a datum circle in the shoulder of the case. There is nothing physical to be seen there, it is just a measured location.

Belted magnum rifles' headspace is the breadth of the belt - or the recess in the chamber for it. Likewise rimmed calibers figure headspace on the thickness of the rim - and the clearance in the gun to accept it.

Everything has manufacturing tolerances and since a maximum cartridge must go into a minimum chamber, there is bound to be a little slop.

Good luck on finding a Net glossary.
There used to be books on paper with such useful information. Most are out of date and out of print. Maybe somebody has a recommendation.

[Johnny Guest]

Not a glossary by any means, but an ennumeration of abbreviations, acronyms, and some definitions:
http://thefiringline.com/Misc/library/TFL_Primer.htm

While The Firing Line is closed for posting activity, it is still on line for reference. The search function works well, and there's a wealth of information in the archives.

macavada, I see you registered back in JAN, but haven't posted on THR much. Jump in more often, huh? :p

Best of luck,
Johnny

[Standing Wolf]

Better some creep than risk a hair trigger on a production line pistol. Your guns probably have some creep and are safer for it.

I don't think so. No finger should ever rest on any trigger until the shooter is ready to fire, and that, it seems to me, is both the ultimate and only reliable safety factor that pertains to triggers. I've found the crisper and lighter the trigger, the more accurately I can shoot the gun. I doubt accuracy has much to do with safety in a bullseye target situation; in a life or death defense situation, however, I'd guess accuracy is a major safety factor.

That said™, there are plenty of people who dislike very light, crisp triggers.

[macavada]

Thanks for the replies. I really learn a lot from this forum!

[Jim K]

I would sort of disagree that "creep" is trigger movement. I would say that it is any discernable interruption in the trigger movement, such as might be caused by tool marks or a rough surface. In other words, a "herky-jerky" movement rather than a smooth movement.

As to headspace, the definitions given by Jim Watson are the normal ones. But headspace cannot be understood except in terms of the tolerances that are necessary in the manufacture of ammunition. All ammunition is made to a +/- dimension because tooling cannot economically be held to exact dimensions in mass production. So some cases will be "short" (at the low end of the allowable dimensions) and some will be "long" (at the high end of the allowable dimensions).

We then need to understand why headspace needs to be kept in a certain range. Since the need is greater in a rifle, here is what happens when a round is fired. The firing pin drives the cartridge forward in the chamber as far as it will go, then the gas pressure expands the thin walls of the forward part of the case, causing the case to grip the chamber wall. The case then backs up under pressure, but the forward part cannot move. The rear part of the case is driven back and the case stretches. If the case can stretch too much, it breaks, letting gas loose in the action and leaving part of the case in the chamber. None of this is good.

So the headspace check works this way.

The GO gauge ensures that the longest case that is within tolerances will fit the chamber and fire.

The NO-GO gauge ensures that the shortest case that is within tolerances will not be allowed to stretch enough to be dangerous.

These two gauges are used at the factory or when rebarrelling a gun or installing a new bolt.

But guns that have seen use will have wear and peening of the locking lugs, breech face, etc., and will not be as they left the factory. The "FIELD" gauge is employed to check headspace on used guns. If the bolt closes on the FIELD gauge with only a bit of play, the gun may be usable for a while. If the gun closes on the FIELD gauge with a lot of play, it is time to have the gun rebarrelled or a new bolt installed. ("FIELD" in this case does not mean out in the meadow, it means anyplace outside the factory.)

If you read in this that only the FIELD gauge is needed to check used guns, that is correct. The GO and NO-GO gauges are nice to have, but perfectly safe used guns will fail the NO-GO test.

Jim

[Sunray]

The Field guage is for testing the chamber/bolt combination to see if it is within maximum tolerance allowed in the headspace of that cartridge. That's the distance between the bolt face and a spot on the case determined by industry standards. If a bolt closes on a No-Go but not a Field guage it's marginally ok. Steel is elastic. It will stretch and compress over time and repeated pressure. A rifle with bad head space is one that has stretched too far. Then you get to have it re-machined to put it back into spec. You cannot fix bad headspace by using longer cases.
Headspace guages actually measure nothing. They only tell you if that chamber/bolt combination are within specs when closed on a sized case, for that cartridge. They are precisely made instruments machined to industry standard sizes. Go and No-Go guages are very common in manufacturing. QC departments are full of them.

[Bob Brown]

Marco: If you want to visually observe trigger creep, here's how. Cock the hammer (on an empty chamber) and pull the trigger back through the 'takeup' until you feel firm resistance. You are now attempting to pull the sear from under the hammer hooks. If you look at the trigger junction where it disappears into the receiver (frame), you'll be able to spot any trigger rearward movement before the hammer falls. If the trigger does move more than a barely perceptable amount, that's creep. It's caused by the sear moving out from under the hammer hooks until it clears the hooks and the hammer falls.
BTW, if the trigger's 'overtravel screw' isn't adjusted properly, or you don't have one, there'll probably be lots of trigger movement 'after' the hammer falls. This makes it hard to spot the initial creep...but not impossible!

Bob

[Jim K]

Hi, Sunray and guys,

Excess headspace can be caused by factors other than a receiver stretching. It can be caused by locking lug seats being pounded, by replacing a bolt/breechblock, or by wear on the locking surface of the lugs.

A field gauge is saying that the headspace is such that a cartridge having a short case (minimum size within the spec) might, if fired in that chamber, stretch enough to separate. A medium or long case may work fine with no problem, since the stretching will not be enough to cause separation.

The "spot on the case" is true for rimless cases which use the shoulder for case support. But for a rimmed or belted case, the headspace dimensions are based on the thickness of the rim or belt. The rules are the same, though.

If headspace involved only the firearm, it could be set to some figure and would be right, just as bore diameter is. But as long as ammunition will not be made precisely, we have to allow tolerances in the headspace, and that is what the gauges do.

Some other time, I will discuss why ammunition has tolerances, but not this time.

Jim

[Gun Plumber]

Actually, the 'FIELD' headspace was originally used for MILITARY firearms. While specs on civilian firearms was considered 'tight', the military wanted firearms that would still function when in 'field' conditions. You know, dirt, dust, mud, etc. Like the AK-47 or the M-14. The M-16 was notorious for NOT FUNCTIONING properly in the field, even though it met 'FIELD' headspace tolerances. Namely because if it was that dirty, other parts that needed to be clean (gas port, etc) were too dirty for the weapon to function properly.

Hope that helps. And they are all correct. Any rifle that will close properly on a field guage is GENERALLY SAFE to fire. HOWEVER, when in doubt, let a smith check it out.