308 Winchester Finishing Reamer Question

[Pirate69]

4-D Reamer Rentals has two choices for 308 finishing reamers. The specs for the two reamers appear to be the same except one reamer is stated to have a 0.335" neck. I do not understand why the 0.335" neck is specified. I have two 308 short chambered barrels I would like to finish. One is a heavy Shilen barrel that I would like to built a precision rifle with. The other is a Brownell barrel that will become a hunting rifle.

I don't understand which reamer I need to rent. I want to give the priority to a reamer for the precision rifle. I sent the same questions to 4-D Reamer Rentals a couple of weeks ago but I have not gotten a response. Guess they figure someone asking this question does not need to rent a reamer.:oops: Any help will be greatly appreciated.

[fouronesix]

Well... a .335 neck in a 308 is awfully tight! It would probably require neck turning and constantly paying attention to neck wall thickness. Common "tight neck" 308 specs are about .340. The "standard" reamer specs for a "standard" 308 neck are a few thousands larger than .340… just to give a relative idea of what the .335 means.

In any case and IMO, I wouldn't go any tighter than about .340 for either application and my preference for a hunting rifle would be a little larger than .340.

[Iron Whittler]

The .335 neck is custom match size. Factory 308 win ammo will not chamber in that tight a chamber. You will have to neck turn all cases to be used in that rifle. This is a roll your own proposition. For a Bench rifle, this is fine. For a hunting rifle, the standard neck size will allow you to chamber factory or reloaded ammo. Also, check throat length as some tight neck reamers will require the use of a throat reamer to get desired throat length for bullet you want to use.:cbpour::swagemine:

[country gent]

You may want to do a chamber cast of the barrels. Using a smaller neck dia reamer than the short chamber reamer was can leave funny looking necks in the finished chamber. if it dosnt clean up the original neck. Short chambered is ussually .100 or less

[fouronesix]

Excellent point about doing a cast of the two chambers. The results will likely determine what neck dimension reamer(s) to use. Hard to say exactly what the short chambers' specs will be. An alternative to the chamber cast would be to drive a tapered, soft lead slug into the existing neck then drive it back out from the muzzle.

Here's the SAAMI specs for the 308 that can be used for reference. Probably have to click on thumbnail to read the numbers.

[Pirate69]

Thanks guys. Good information. I compared the two reamers to the SAAMI specs and I now know which one I need.

[Pirate69]

Couple of follow-up questions. I have noted that some reamers are fixed pilot and some are floating pilot. What is the difference in application for these two types? Does it make a difference which one I use? I really would like to use a pull through reamer if I can find one.

[fouronesix]

The pilot on a reamer is about bore diameter and it's what "leads the way" for the cutting portions and keeps the front end of the reamer lined up with the bore.

The fixed pilot is just that and sized to fit, in this case, a bore diameter of .300- a kind of one size fits all thing. A 30 cal fixed pilot will not be exactly .300- it's probably a little less and the precise diameter of the bore is probably not exactly .300 either. A floating bushing reamer has an interchangeable, freely rotating bushing of bore diameter that is designed to rotate on the reamer spindle and not rotate on the bore surface as the reamer turns and moves forward during the cut.

A floating bushing reamer is generally considered better because: it runs no risk of scuffing the bore ahead of the reamer and individual bushings can be used to precisely match the bore diameter. Those bushings are usually available in .0002" increments for that precise fit.

[country gent]

Both are used the same way and interchange. there is less chance of scoring the throat and other damage issues with the floating piloted reame as it turns with the barrel sliding straight in. Another plus is pilots can be changed easier to fit the bore with the running snug fit than the fixed piloted reamers can. I perfer the piloted reamers for almost all chambering work. Proper Fit is everything at the pilot. to tight and you score, gouge or damage the first part of the rifling to loose and it floats around. I also recomend alot of good quality sulfar cutting oil. If machine chambering then cutting oil pumped thru barrel to reamer will push chips out and cool clean and clear reamer much better. Otherwise alot of oli and clear/clean often. What is the reason for wanting to use the pull thru reamer? On bolt actions it is as quick and easy to pull the bolt and use a tap extension thru the action.

[Pirate69]

I was under the impression that a pull through reamer was the best choice with regard to ensuring that the reamer was centered.

I cast both barrel chambers with Cerrosafe. I used a dial caliber to measure the casts. I zeroed the dial and checked a couple of 30 caliber Sierra bullets. As expected, they read 0.308". This was the best I could do to verify my caliper. Multiple reading of the Brownell barrel gave a 0.3390" result. Multiple reading of the Shilen barrel gave a 0.3480" result. These readings were taken over an hour after the casts were removed from the chamber. Looks like the SAAMI spec is 0.3435". Is this the range of variation that I should expect? Looks like the Brownell barrel may be a little tight. Looks like I may need to rent two reamers with different pilot sizes? Comments please.

[country gent]

On a .308 caliber barrel bore dia should be around .301 It looks like you measured neck dia from your dimensions. Pilot dia needs to be bore dia as it will be the first portion of the reamer, ahead of lead throat neck and shoulder. If the pilotis fit to neck the reamer will never be able to cut it as the pilot will be there stopped buy the rifling or start of throat. The bore dimension can be measured several ways a chamber cast that is into the rifling, a lead slug pushed thru the barrel from breech to muzzle part ways then back out, or accurate pin gages. The pull thru reamers dont always cut the "whole" chamber but the shoulder neck and throat. They were made for armorers where the shoulder only had a small amount to be removed

[Pirate69]

Oh well, back to the garage.

[fouronesix]

Those neck diameters you measured will require two different reamers if you want to keep a fairly tight .339+ neck in the short chambered barrel for your precision rifle. You'll need to order a reamer with that neck diameter up to maybe .341. Then a different reamer for the fatter neck short chambered barrel. The only option for one reamer for both would be to settle for a reamer matching the fatter neck, but then both chambers will have the fatter neck.

Pilot needs to fit the bore- has nothing to do with the neck. With new, quality barrels I imagine the bores will be a smidge larger than .300- especially if they were lapped after rifling. The only way to precisely predict the best pilot bushing size would be to measure the bore diameter to the .0001 level of precision and get a bushing about .0001-.0002 smaller. Given average or even good tools and methods… that is easier said than done. It would be better to get maybe 3 best guess bushings in .0002 increments and try them in the bores by feel. One bushing matching the bore diameter erring on the small side, then two more smaller in .0002 increments. They can be had separately and are simply mounted on the reamer pilot spindle with a single end screw. Then it's a matter of using the best bore fit bushing on the reamer used in each bore.

[Pirate69]

The Shilen barrel is the barrel with the fatter neck (0.348"). The hunting barrel is 0.3390". Correct me if I am wrong, but if there is any slop, you would want it in the hunting barrel and the best fit in the target barrel. Are there any drawbacks with using the best fit reamer, for the Shilen barrel, in the smaller neck barrel? I am still working through all this in my mind, so pointing out errors in my thinking is a major benefit to me. The first thing I am going to do at this point is to redo the casting to confirm what I think I am seeing. Don't want to mess-up anything I can not eat.

[fouronesix]

You're thinking is correct. But seems the short chamber necks in the barrels are backwards from your original plans- where the heavy barrel for the precision rifle has the fat neck and the hunting barrel has the smaller neck. Is that correct?? The hunting chamber should have the fatter neck.

No matter which short chamber you ream- the finish reamer neck has to completely clean up the existing neck otherwise there will be a "step" in the neck…. not good. For example: you order a reamer for the short chamber that has the 448 neck, it would be best to order a finish reamer with a neck that is at least 449. That will be the best chance of avoiding any step left in the neck. The same would hold true with the tighter neck-- unless you go ahead and decide to use a single reamer with a .449 neck for both chambers.

The other option would be to shorten the fat neck barrel by the length of the existing short chamber neck. Then use any neck dimension finish reamer you want. But that will require some parting, adding a few threads and refacing the receiver ring shoulder on the barrel.

Don't be confused by my references to the pilot bushing diameters taken to .0001. It's just better to try and get a pilot bushing with as little bore run out as possible. That is the one thing in the process that can be easily controlled from the beginning. The rear of the reamer is the hard part and that's where the runout is difficult to control- no matter how well the barrel and reamer are dialed in…. even in the best of lathes.

[country gent]

Its not always easy to decide what you want or need. A pilot to fit bore closely helps to keep the reamer alighned with the bore, Hopefully the short chambers are cut straight and true with the bore or the existing chamber and pilot will fight each other some. A lead slug driven into the bore from breech a few inches forwrd of throat and back out will give the barrels bore and groove dimensions you wantto order the pilot for the reamer. -.0002 --.0005 is a good fit to aquire this your michrometers and the reamer makers have to measure the same in the same ampient temps. Some pilots are help on with screws some with a small e clip ( manson reamers). Slug the bore at the breech end package it up in a solid tube boxed good and send it to the reamer maker or rental company if they will work with you and let them measure both on site. This takes diffrent temps measuring tools and peoples feel out of the equation. The necks need to clean up from existing to the new cut how long the new cut will be depends on how short the chamber was left. The important thing is to get everyyting right before starting cutting. Measure some loaded 308 rounds neck dia and add .001-.002 to the largest this will give an idea of needed neck dia. The neck has to have a little extra room to release the bullet. Tight neck reamers are out there that require the necks to be turned to maintain a vey consistant clearence. While sami specs give dimensions many have thier ideas as to what is needed and improvements

[Pirate69]

I did get a chance to re-cast the barrels. No change there. I would hope not. I will slug them when I get back from MI. Just checked the conditions. Looks like 20F with a 3F windchill. Going to be cold. Many thanks to those that are coaching me through these installs.