Gunsmithing ? On PTG Bolt Body

[Wannashootit]

First time ordering a custom (two piece) bolt body from them.
Magnum boltface, M16 extractor cut, .698 diameter.

They skim cut the full diameter of the body outside the area of the lugs, behind and on both sides of them- and not ahead of them. Meaning, the bolt nose is not round as can be seen in the pictures. What's the purpose of this? I can't attribute the need to do this to anything- and I obviously need to use the full diameter dimension when calculating what's needed for the diameter of the bolt nose recess.

I normally shoot for .005-.008 clearance per side on the recess (yeah, I know it's on the "large" side, I don't see a reason to keep it tight) .
What's needed for proper function of the extractor?

 

[Mordamer]

Measure maximum bolt nose diameter. Let's just assume it is .700".

Now install the extractor and stick something round like a gauge pin in the bolt nose so it forces the extractor out. Now measure from the opposite side of the bolt nose to the outside edge of the extractor to see how much the extractor might encroach outside the original .700" measurement. If your measurement across the bolt nose/extractor while it is forced outwards is .720" then there is a chance your .716" counterbore (.008 clearance both sides) isn't enough.

Substitute in your measurements. It sucks to have to recut the counter bore because it wasn't a large enough diameter and the extractor can't pop over a case rim.

 

[Wannashootit]

It sucks to have to recut the counter bore because it wasn't a large enough diameter and the extractor can't pop over a case rim.

Yessir, thanks- exactly why I asked.
I'm thinking I'll turn a piece of stock .549 to fit the .550 boltface, rather than try to keep a "rim diameter" (.535) pin centered which might be more difficult to do, then take the measurement; the larger plug will provide some additional side clearance since the .535 case rim won't push it out as far.

But I'm still wondering why they turned down the body between the lugs.
Maybe part of the machining process of the lugs themselves based on programming for standard bolt diameter (?)

 

[Mordamer]

The surface finish almost looks like a grind or polish rather than turning marks. Maybe they do that to make the bolt slide back and forth in the channel smoother.

If you don't mind, after you take your measurement could you post an update here? I'm interested in how big the counter bore will need to be. Every Rem 700 I have done was still a remington extractor.

 

[Wannashootit]

The surface finish almost looks like a grind or polish rather than turning marks

Yeah- poor terminology on my part, couldn't have been turned since it's interrupted by the lugs. Like you said, or could've maybe been done on a CNC machining center- above my pay grade

I'll post up the measurements after Brady and Gronk...

 

[iceng]

It's part of the manufacturing process they use. It's not visually appealing, I get that, however it's not detrimental to accuracy or function.

 

[jetmd]

Just an FYI:

My buddy just had to return a PTG Magnum bolt back for warranty/repair as the bolt face
measure .552-.553
Specs are .539 min - .544 max
PTG actually took care of him....only good thing I have ever heard or know about PTG.
Anything over .544 and you are asking for difficult operation.

Hope this helps.

 

[Wannashootit]

Anything over .544 and you are asking for difficult operation.

How so?
Not following why .550 (spec, per their website- I haven't been back in the shop to measure yet) would cause "difficult operation", with the bolt nose recess cut for adequate clearances accordingly?

 

[mcfred]

But I'm still wondering why they turned down the body between the lugs.

It's part of the manufacturing process they use. It's not visually appealing,

I agree it's an artifact of their manufacturing process.

It appears that they remove the area between the lugs in a mill with a cutter that's in the profile of the lugs and some minimum bolt body radius; roughly an hour-glass shape. PTG probably leaves the bolt body OD fat, to be ground down later per customer request. I bet the lug-cutting operation is quick and does not leave an appealing surface finish, so they bead blast it to hide the rough marks, and put a one-size-fits-all part on a shelf waiting to be finish-ground. When you call and order a Ø.698" body, the lug milling marks do not clean up like the rest of the body.

 

[Wannashootit]

Finally got back to chambering this barrel after the holidays.
I had thought I would need much more clearance in the bolt nose recess for the M16 extractor, but measurements don't bear that out.
Measuring across the largest diameter (it's not symmetrical on the outside due to how it's cut), it's .6983.

Turned an acetal plug to .539, placed it in the boltface then inserted the extractor and miked across the bolt nose.
Only a few thou more- .701.

I'm puzzled because Badger Ordnance specs .785 for bolt nose recess diameter.
What's different about their design that needs over .035 (per side) additional for extractor clearance?

I'm planning to cut as I normally do (large)- twenty over on diameter at .721. Can't see how I'm missing anything, but...

 

[Hateca]

Are you accounting for the extractor riding over the rim? Not just after the extractor has seated over the case rim. Depending on the extractor, caliber, bolt nose counterbore, and any relief cut into the counterbore the extractor can hit the face of the barrel as the extractor try’s to snap over the rim. Just depends on the caliber and extractor.

 

[iceng]

As @Hateca said. You need to allow the claw to ride over the rim before it goes into the groove.

Cut the bolt nose recess over size. Once the bolt is in position, it doesn't matter and won't effect accuracy. Don't go stupid, but definitely clearance it.

Also that's clearance measured for one side, you need to account for the other side too. Radius vs diameter.

 

[Wannashootit]

As I mentioned, the "plug" was cut full diameter of the boltace. I did not use an actual case with a rim of lesser dimension. Extractor cannot be pushed further out by anything that would fit into the boltface. It is a rebated rim with larger base diameter but that is part of the chamber not the bolt nose recess.

 

[Rubicon Precision]

As far as why the "skim cut", I'd bet they rough machine then heat treat in batches. Then the finish grind the dimensions that matter when the customer orders.

 

[LRI]

As far as why the "skim cut", I'd bet they rough machine then heat treat in batches. Then the finish grind the dimensions that matter when the customer orders.

I admire your optimism buddy. My bet is they buy "prehard" Chromoly at around 34-36 Rockwell and send it. The stuff machines way too easily to be in the 40's. I've not ever "dinged one" on my hardness tester to be sure, but I'd still throw a lunch date at it on a bet.

OP:

This may have been stated earlier but my first question is why is the extractor cut "stepped" in your photo. I've machined thousands of bolts for M16 extractors and never once seen the feature done that way.

As for the "lumps" on the bolt nose, if this were here I'd likely throw the bolt body in a collet and whisker the nose until they caught up and blended to the circumference of the bolt body. There's no rational explanation for why they got left like that.

Bottom line is I think your suffering from PTGitis.

Good luck on your project.

C.

 

[Threadcutter308]

I admire your optimism buddy. My bet is they buy "prehard" Chromoly at around 34-36 Rockwell and send it. The stuff machines way too easily to be in the 40's. I've not ever "dinged one" on my hardness tester to be sure, but I'd still throw a lunch date at it on a bet.

OP:

This may have been stated earlier but my first question is why is the extractor cut "stepped" in your photo. I've machined thousands of bolts for M16 extractors and never once seen the feature done that way.

View attachment 7532829

As for the "lumps" on the bolt nose, if this were here I'd likely throw the bolt body in a collet and whisker the nose until they caught up and blended to the circumference of the bolt body. There's no rational explanation for why they got left like that.

Bottom line is I think your suffering from PTGitis.

Good luck on your project.

C.

Awwww.........c'mon man, after the extractor's installed, you'll never see it......

(/sarc)

 

[Wannashootit]

This may have been stated earlier but my first question is why is the extractor cut "stepped" in your photo. I've machined thousands of bolts for M16 extractors and never once seen the feature done that way.

Chad,
Good eye. Didn't even notice it, "PTGitis" is appropo...

Chamfer at the end of the extractor cut. They failed to take it all the way to the edge...

if this were here I'd likely throw the bolt body in a collet and whisker the nose until they caught up and blended to the circumference of the bolt body. T

Yeppers. Hindsight is 20/20. I wasn't concerned about the aesthetics of it, but...
Unfortunately, I made the error in "assuming" the lugs would be square to the bolt body (at least, good enough to need a few strokes of lapping compound at most. I sent it to Dan at Accu-Tig for the new handle (excellent work- for those that need it done) as soon as it came in.

This was supposed to be a .260 hunting rifle build. Customer- my son in this case - changed his mind to go with a .300 WSM after I'd gotten the action with std. boltface. Hence, the new bolt. I'd already trued the abutments and receiver ring (as well as the factory bolt- which was now useless)- when I got the bolt back with handle installed, put some Dykem on the lugs and got near zero contact- one edge. So, had to take this precisely timed new bolt- indicate it in and face the damn lugs, losing some five thou primary extraction in the process. Got it lapped in to 100%, didn't lose too much- but lesson learned about the level of "precision" on their bolt bodies.