Back Bored rimfire barrel

[Hi-NV Shooter]

Lee,

That's true if the bolt fully chambers the cartridge. But what if the case rim is thinner than the headspace? Surely then, the bolt will only chamber the cartridge so far before it locks into battery.

Tim, I think you missed that I wrote any further once it seats against the breech face. so all you have to play with is adding more more headspace. and do you really believe adding more HS is good? at worst you alter pin hit depth.

Lee

 

[Hi-NV Shooter]

Tim you hit the nail on the head. Matter of a fact the bolt can't fully seat the the rim to the barrel face. There is so much variance in rim thickness some rounds would be smashed when the bolt closed. That is why we have "headspace". The headspace always has to be larger than the rim thickness. The question really is how much larger? That depends on the ammo/lot you plan to shoot. Eley normally runs about .038 -.039. Lapua about .039- .0405. Generally, you need a little more headspace for Lapua to ensure you aren't crushing the head and having hard bolt closing.

With all this said, I still haven't answered the question of how much headspace do we need.

Years ago Time Precision made rifles that had easily adjustable headspace, and I spent years and a lot of money adjusting headspace.

I would sort my ammo by rim thickness then adjust the headspace accordingly. I tried everything from slightly crushing the head to so far out the round wouldn't fire. I never found the answer. The only thing I learned was too little headspace doesn't work as well as more headspace.

As a matter of fact as long as the rifle fires 100% of the time you don't have too much headspace. But if the bolt is hard to close on some rounds you don't have enough.

There was a time in RFBR .042 was considered the best headspace, then it went to .043. Now most are using .044 or greater.

The takeaway from all of this should be if some rounds chamber harder than others you don't have enough headspace for that brand or lot of ammo.

I'll also suggest a little test. Try pushing the round all the way to the barrel face with your thumb and shoot groups. Then just push the round in the chamber and let the bolt push it the rest of the way and shoot groups. You may be surprised.

TKH

Thanks Tony, as always your knowledge is greatly appreciated. my point was that once fully seated the bullet can not go in any further so engraving will be what it is at that point. now if 0.044 is now the happy medium for HS for the reasons you mentioned, would be it a good idea to increase HS to alter what little engraving it may change and lose pin hit depth? since setting up good ignition needs to include the HS.

Lee

 

[Hi-NV Shooter]

Your 100 yd group ( if the 3 shot group) is not enough shots to get any significant data . 10 shot or 20 shot groups repeated 5 times . Then your data will be relevant. My 20 shot g
may I suggest that we take this to a separate post , we’ve gotten the original post off track .
I didnt say it wasn’t improved at 50 , it was much more difficult to measure the small improvement , by going to 100 and further you can get better data , easier to measure due to the group getting larger
Your assumption about good at 50 good at 100 is just with your ammo , gun on that day . For instance, take a high vel ammo , great at 50 crappy at 100 , going transonic. Or a border line std vel ammo thats shooting faster than it usually does , due to weather , bore , barrel length etc, doing the same thing , going transonic before the 100 yd mark .
one test with one ammo with small lots isnt good data

The data I rely on are targets like this, that is my goal to have ammo and rifles that can produce these types of targets come match time. groups are a baseline for me to see how the rifle is shooting. then if they can produce scores like this I don't need to alter anything and would be foolish to change anything.
the targets are from 2 different rifles and 2 different lots of Lapua CX & Pistol King

Now I will ask one last question. if you believe testing at 100-300 will show that what you are doing is working, how do you control what the wind drift may do at the target, how can you be sure where POI occurred is truly the results of altering HS and not the wind!

Lee

 

[Pyscodog]

I finally got a chance to shoot the H&R this evening. I had a club meeting and not a lot of time so I only took a partial box of cci standard . This is 11 shots at 50 yds.

 

[Hi-NV Shooter]

I finally got a chance to shoot the H&R this evening. I had a club meeting and not a lot of time so I only took a partial box of cci standard . This is 11 shots at 50 yds.

shows promise, not too bad with just CCI better ammo may give better results. at least it is not shooting like a shotgun

Lee

 

[Pyscodog]

It was a little breezy, misting rain and I was short on time. I mainly went to see if it would at least function. I was pleasantly surprised with the results. You never know with a pawn shop purchase.

 

[Tim Slater]

Tim, I think you missed that I wrote any further once it seats against the breech face. so all you have to play with is adding more more headspace. and do you really believe adding more HS is good? at worst you alter pin hit depth.

Lee

Lee,
that's true, but not exactly what you wrote earlier You may have meant " How is changing headspace effective beyond the point the cartridge is fully seated?", but you didn't say that. You just wrote adjusting headspace with no qualification.

 

[Ggmac]

Ggmac,

I'm not sure what you are saying.

It is for sure I can't measure .0005 1/2 of thousands.

If you are shooting rimfire, that is a good thing.

Good luck with your testing/shooting, hope to see you at a match sometime.

TKH

The data I rely on are targets like this, that is my goal to have ammo and rifles that can produce these types of targets come match time. groups are a baseline for me to see how the rifle is shooting. then if they can produce scores like this I don't need to alter anything and would be foolish to change anything.
the targets are from 2 different rifles and 2 different lots of Lapua CX & Pistol King

Now I will ask one last question. if you believe testing at 100-300 will show that what you are doing is working, how do you control what the wind drift may do at the target, how can you be sure where POI occurred is truly the results of altering HS and not the wind!

Le

Ah , I think it what Im calling a group , sorry , my group is One shot at one individual 1/4” dot , ten or twenty times .so each group would have 10 or 20 1/4” dots with a shot at each . That way I can evaluate each shot , and get the data that ends up represented by the bell shaped curve .Doyou understand the bell shaped curve and removing the high and low from the curve ? These two will remove the the variables your asking about . If you’re shooting in a windy condition, the groups would be shot with 20 shots per group , repeated 5 times . Remove the great and worse . This whole conversation has me rethinking the reason headspace matters ... you know how just finger pressure on the barrel can change the poi , what if all I was doing was increasing the pressure on the base of the cartridge, therefore changing the harmonics? I dont know if I expressed my thoughts correctly but Im trying to think of a way to test this , if possible .

Its been a WHY does it do question to me , and competition really isnt my game anymore , well not until I know the why . Continuing the testing , eliminating as many variables as possible, I tried shooting rests ( machines) and may have to go back to them . Coffee time

Lee,
that's true, but not exactly what you wrote earlier You may have meant " How is changing headspace effective beyond the point the cartridge is fully seated?", but you didn't say that. You just wrote adjusting headspace with no qualification.

Not all bolt designs will change fp protrusion with headspce change . It depends on type and where the firing pin stop is located . Needto check my notes on this .

 

[Ggmac]

No coffee yet and somewhere I had an explanation, now I cant find the couple of paragraphs I wrote , ive got my morning friend on my lap , a cat that wants to get food . So somehow its missing , Ill try to answer Lees question.

 

[Ggmac]

Try again , im think my first attempt somehow is under Lees heading?
At first, my testing needed the range to eliminate the highs and lows ( ins and outs ) of the group when shooting at the same poa . Then I changed to shooting one shot at one 1/4” dot , it made it easier to get accurate data .
Havent you ever wondered why one lot of ammo shoots better ? Could it be so simple as being a headspace issue or could headspace be at least one part of the equation ? What if it is , how do we determine if it is a factor, by testing. Just my thoughts. Some just go on and try toget the lot that shoots the best in the gun they’re using and at the range they’re shooting . Thats fine , but if we are to make improvements in the sport , its my belief that testing is necessary.

 

[Tony K Harper]

Thanks Tony, as always your knowledge is greatly appreciated. my point was that once fully seated the bullet can not go in any further so engraving will be what it is at that point. now if 0.044 is now the happy medium for HS for the reasons you mentioned, would be it a good idea to increase HS to alter what little engraving it may change and lose pin hit depth? since setting up good ignition needs to include the HS.

Lee

Lee,

This whole subject is far more complicated than I have words. Headspace, firing pin fall/ case head protrusion, trigger timing, chamber depth/bullet engraving, twist rates, ammo sorting, etc., etc.

All of these things have been experimented with by countless people for a very long time. When you start down this rabbit hole one thing you learn is they are all interrelated. Every one of them affects every other.

When setting up a new build it all comes down to a balancing act. An art as much as a science. Some even believe the barrel/chamber is best set up for a particular lot number of ammo. Small pictures are easier to draw than larger ones.

In the final analysis targets/match results tells the story of how successful your overall system (rifle) is.

I believe rimfire rifles can not be shot with much precision beyond 50 yards/meters. Testing at longer ranges is useless. There is just too much to deal with to tell if the shooter or the rifle is the problem.

Perhaps we find this stuff interesting only because no one has found the final solution.

TKH

 

[grauhanen]

For what it's worth, and as an observation only, Calfee said in his book and elsewhere that headspace had nothing to do with accuracy so long as ignition was very good. In 2016 he said he continues to recommend using .043" as the headspace.

 

[VLNBYR]

The Trifecta...3 rabbit holes going on at the same time.

1..Sorting by rim thickness
2..Headspace and accuracy
3..Long range shooting