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Barrelling- Tight Necks

 
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  #1  
Old 01-27-2005, 10:41 PM
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Barrelling- Tight Necks

I have a 243 Ackley that I am looking to rebarrel. I have read about tight neck barrels but have never had one. I have turned necks and have all the proper measuring equipment. What I have no doubt read before but forgotten is the numbers part of the game. I would like to know how many thousands clearance is needed (ie safe) between a loaded round and the chamber. Maybe .002" total, with .001" on each side? Also, some make brass to have a tight fit and claim they never have to size brass, what would the clearance on this be?

Anyone with experience in standard vs. tight neck chambers in custom barrels please chime in. I would like to hear all experiences.

I plan on using a #5 fluted Pac-nor 27" stainless on a M700 short action with a 1-12" twist to shoot 70 and 75 ballistic tip and VMax type bullets. Rifle will be pillar bedded with aluminum pillars and Marine-Tex, with floating barrel. I would like to have rifle shoot in the 4's if possible, and wonder if going with the tight neck is needed. I don't mind doing the extra work if it can significantly shrink my groups, like maybe 0.1" @ 100 yards.

Thanks for the imput fellers, appreciate it.
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  #2  
Old 01-27-2005, 11:31 PM
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Re: Barrelling- Tight Necks

Zane,

I like to tell my customers that for the tight neck chambers I cut for them, they need a MINUMUM of 0.0015" total clearance between the bullet and neck upon firing. That means 0.00075" on all sides.

This is as close as I feel is safe. A better clearance dimension is 0.002" total as you list which allows 0.001" clearance on both sides.

This just gives you a bit more safetly margin especially if you are using a high intensity round.

Tight necks are great for accuracy but perhaps even more important is a properly designed leade and throat. A tight throat will aid accuracy more then a tight neck from what I have seen. In my Varmint, target and V-Block rifles I will cut the throat to a very tight 0.0003" over the bullet diameter my customer wants to use. This is important as same caliber bullets from different manufacturers can vary up to 0.001" in diameter so 0.0003" over one bullet diameter may be significantly different then to another bullet diameter.

If my customs want to be able to shoot any bullet they want, I will go 0.0005" to 0.0006" over nominal bullet diameter.

As long as the neck run outs and bullet run outs are in the 0.001" range or less, and your throat is tight and straight, you will get match grade accuracy.

With a properly built rifle, your accuracy goals are well within reach.

Good Shooting!!!

Kirby Allen(50)
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  #3  
Old 01-28-2005, 01:50 AM
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Re: Barrelling- Tight Necks

Thank you very much. Couple of questions. I had the leade angle cut at 1.5 degree as the smith said this is more accurate than the 3 degree. How does leade angle and depth of the throat, (bullet diameter) go together. Are you saying that the throat is cut .003 over bullet diameter, and on a certian angle, like 1.5 degree. I am just trying to learn as much as I can so that on future rifles I can really get what I want.

Also, my concentricity is running about .002 and I can't seem to get it any straighter. I may try a shallow cut in the case mouth (turned) as mentioned in another post on concentricity. Right now I'd say they average .002, with the extreme being .004 on the high end. I use redding bushing neck dies, with no expander plug, and an RCBS seating die. I had a Wilson type in line seating die and the conc. was not any better. The chamber on this barrel may not be within the .001 that is accepted as the fired brass has about .002 to .0025 runout before sizing.

Is the Bersin Tool the cure all to conc? Is the level of conc. that I am currently getting acceptable? I dunno. I test each loaded round for conc, and mark anything over .003. I have not, in 200 rounds been able to conclude that it is the cause of fliers. Like I said though, none are what I would consider terrible, with the extreme being about .004".

Thanks again.
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  #4  
Old 01-28-2005, 06:54 AM
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Re: Barrelling- Tight Necks

zane

.001 is the min you should go, especially with a hunting gun. Normal 1k BR guns are not set any tighter than that and they are meticulously cleaned. No need to go tighter. key is dies that are true and matched to chamber.

For hunting gun, you might consider just min saami chamber and special reamer cut for no neck turning for your brass. Dave Kiff (Pacific Tool) normally sells the reamer for $135.

Bersin tool is waste of money in my opinion.

rotate the case after seating 180 and reseat.

Suggest you look at a Broughton barrel in his 5c rifling. Will be faster and easier to clean. He is making one of the best barrels right now. Second, lot of talk right now about poor quality of the steel PacNor is currently using.

Contact Jim Carsenson of JLC precision (go to 6mmbr.com and look at his info) about a custom FL sizing die with neck bushings cut to your chamber. He uses 10 pieces of fired brass, redding body bump die and about $78. Jim will also tune a redding or forester BR seater to your bullets.

.1 group is lofty goal for non BR gun. .1 in a BR gun is winner every day. Occasional .1 maybe, routine, not likely.
Suggest you find a good 1k BR smith if that is truely your goal. Check recommended gunsmiths at 6mmbr.com also.

Good luck.

BH
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  #5  
Old 01-28-2005, 10:18 AM
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Re: Barrelling- Tight Necks

Zane Shepherd,

The angle into the rifling, depending on how the throat is designed will effect how long you can seat teh bullets out.

For example, if you take a throat with a parallel diameter section(some call freebore) of say .110" in length and with a 1.5 degree angle into the rifling, you will be able to seat your bullets to a certain length.

If you take that same throat but steepen the angle from 1.5 to 3 degrees you will have a slightly shorter OAL because the steaper angle will engage the bullet sooner then the shallower angle.

As far as accuracy goes, I would say you would be hard pressed to prove a 1.5 up degree is more accurate then a 3 degree if both are set true and accurate.

What will be effected is the amount of pressure that will be required to start the bullet into the rifling. The steeper 3 degree angle will increase chamber pressure quicker then a 1.5 degree angle simply because full bullet engraving of the rifling will take place twice as fast as with a 1.5 degree angle.

This is also generally not a problem. It may be only if you have an absolute max load developed for a 1.5 degree angle and then had a barrel made with a 3 degree and used the same load.

Peak pressure would certainly occur quicker and may be higher as well but I would not expect anythign to get dangerous from this.

BAck to the accuracy issue. In high intensity rounds using thin jacketed match bullets, the 1.5 may produce tighter groups compared to the 3 degree simply because the 1.5 eases the bullet into the rifling in a less severe manor.

The thin jackketed match bullets would probably perform better with the 1.5 degree.

For big game bullets I would not expect to see much difference at all to be honest.

I am sure your throat is not 0.003" over bullet diameter unless you have an angled throat right from the leade into the rifling. This design is basically only a cone that guides the bullet into the rifling and often a larger starting diameter is used for these type of throats then with the type I discribed above.

Personally I do not like the conal throats, feel they do not compare in accuracy with the type discribed above but again you know what they say about opinions.

Your concentricities are running alright. I have shot some extremely tight groups with run outs in the 0.002" range. We are always looking to get sub 0.001" run outs but do not discard the 0.002 and 0.003 ammo unless your match shooting.

You really need to test your ammo run outs throughout the loading process. Size your cases and see what the neck run outs are before you do anything else. YOu need neck run outs in the 0.001" range or less if you want any chance at all to get at or under 0.001" bullet run out as generally the neck run out is magnified in the bullet run out.

If youare using an S type Redding bushing die I would highly recommend you get a Redding Comp Neck Bushing die. The S die is a great die but is does have one flaw, it does not support the case as the neck is being sized. While you have greater control with the bushing as far as neck tension goes, you have no more accuracy then a standard full length sizing die as far as controlling neck run outs in your case.

The Comp die from Redding hold the case perfectly centered as the neck is sized resulting in 0.001" or less neck run outs nearly all the time is you make sure to keep debris out of the bushing area of the die.

Another problem using the bushing sizing dies that most do not realize is that they make the outside of the case neck concentric but unless you turn your case necks, all the case neck thickness variation is transfered to the inside surface of the case neck. The outsides are nearly perfect but if there is any thickness variation in the case neck, it has all just been transferred to the inner diameter of the neck, Where the bullet will be held.

So unless you are turning your necks to get them perfectly even in thickness, do not expect to get sub 0.001" run outs a high percentage of the time because the neck thickness variation will be carried over to the seated bullet.

Once you have your cae necks sizing true its time to go to the seating die.

First though, you need to make sure your brass is trimmed square to the axis of the case and also that you have an even chamfer on the inside of the case mouth.

This is because if your case necks are not square, one side of the bullet will engage teh case mouth before the other and when the seating pressure is applied, the bullet will cant slightly until the entire bullet base contacts the case mouth, this means higher run outs.

IF it is square, the bullet will start to seat square and you have won half the battle.

The chamfer is also critical as an uneven chamfer will do the same thing an uneven case mouth will.

So now we have the case neck run outs under 0.001" and the case mouth square and chamfered squarely. Now its onto seating that bullet true.

Take a dumby case that as a know run out, preferably under 0.001" and seat a bullet in it as you would if you were topping a loaded case. Pull the bullet and check its concentricity. Generally, whatever your neck run out is, you can double that for your bullet run out or more if your necks are not turned.

Doing a few of these will tell you the consistancy of your seating die. If you get run outs all over the board something is not consistant with the seating process. Generally, as with the sizing operation, the case is not being controled and held in line as the bullet is being seated.

Again, teh very best system I have used is with either a Redding Comp seater die or a Forster BR seating die. Both use the sliding sleeve, in-line system as does the Redding Comp neck sizer.

The sleeve holds the entire case, not just the top half, the entire case length in perfect concentricity with the bullet as it is being seated.

With these dies, I have found that bullet run outs on turned case necks will often be equal to or even slightly less then neck run outs instead of being magnified using a stardard seater.

These dies do cost quite a bit more but once you use a set, you will forget about any price tags when you measure 100 rounds and the highest bullet run out you have is 0.002" and 80% are under 0.001" run out.

One thing that really concerns me with your problem is the fact that you are getting 0.002 to 0.0025" neck run outs on your fired cases, this is not acceptible at all.

Sometimes the Rem 700 can be a little rough on the case mouths during extraction and ejection leaving a flattened area on teh case mouth. Are your cases doing this and your picking up the run out from this?

If not your chambe is dramatically off. Perhaps not off axis but either it is off axis or your chamber is egg shaped.

Was your chamber cut with a reamer that used a live pilot bushing design or a solid pilot? THe only way to cut a match grade chamebr is with a live piloted reamer that has interchangable pilots so youcan perfectly fit the pilot to the bore diameter. Thes come in different sizes for each caliber.

For instance my sets have 9 different sizes for each caliber, each 0.0002" larger then the one before. When chambering or crowning a barrel, I use the pilot that perfectly floats on the lands. Using these correctly, there is no way to get a chamber off center if everything else is also done correctly.

TO check to see if your chamber is off or egg shaped. Use your concentricity guage to measure the concentricity of your case body right at the body-shoulder junction as well as at the rear of the case, but make sure it is above the expansion ring or you will get false reading.

IF these measurements are true, I would say your getting the run out from the ejection process. IF your chamber is egg shaped, youwill see this in the variation in these two measurements. If it is egg shaped, you will either need to live with this rifle as is or have it either set back and rechambered or rebarreled.

As far as the Bersin tool is concerned, for the money by a set of Redding Competition dies and true your necks and forget about run outs being a problem.

Hope this helps some,

Kirby Allen(50)
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Kirby Allen(50)

Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.

Farther, Faster and Flatter then ever before.

Web Page: www.apsrifles.com

allenmagnum@gmail.com
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  #6  
Old 01-28-2005, 10:36 AM
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Re: Barrelling- Tight Necks

Fifty,

[ QUOTE ]
First though, you need to make sure your brass is trimmed square to the axis of the case and also that you have an even chamfer on the inside of the case mouth.

[/ QUOTE ]

I trim my cases in a standard Forester case cutter which uses collets to hold the case head. I <font color="blue"> assumed </font> that this was leaving the cases squared. Am I wrong to assume this?
[img]/ubbthreads/images/graemlins/crazy.gif[/img]

I also chamfer and deburr cases with a RCBS tool that is secured in a Sinclaire tool holder designed to allow you to spin the cutter with a cordless screwdriver. Basically the alignment while cutting is by eye and hand. Is this a bad practice?

What methods or equiptment do you prefer for these tasks. That is without going nuts, of course. [img]/ubbthreads/images/graemlins/grin.gif[/img]
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