6.5 Grendel PTG Reamer

[BjornF16]

I don't really know if this has any bearing on the subject but I am curious so I'll ask. Again back tracking to a few months ago and the short chamber issues with my McGowen barrel. I made a call to PTG and talked with Dave about my specific problem. Dave was aware of the problem and stated that the Grendel was a double throated reamer. Dave also said to use a .264 LBC reamer and that reamer works just fine and much better with the Hornady Grendel ammo because this reamer uses a parallel throat. What's the difference between a double throated reamer v/s a parallel throated reamer?

Instead of the Grendel double angle throat (0.5 deg which transitions to 1.5 deg), the LBC apparently uses a parallel free bore of .153" (constant .2645" diameter; Grendel uses 0.5 deg over this length) before transitioning to the 1.5 deg angle "forcing cone".

ETA: The LBC neck diameter is .295" vice Grendel .300"; LBC lead is .120" long at .2644" diameter; with a 1.5 deg throat. http://www.thefirearmblog.com/blog/2...grendel-clone/

[biggdawg]

What it sounds like to me is ptg has a problem making the reamer to grendel spec. just my opinion

[cory]

Ted. The bullet in a SAAMI spec round needs to have .020s jump to the lands in a SAAMI spec chamber. If it does not it will cause pressure problems. The throat in most, if not all, liberty barrels is short. If you reload and seat your bullets deep there is not a problem. In my mind every barrel shound go back and have the throat reamed so factory ammo keeps to the pressure limit of the Grendel.

This isn't entirely true. While the SAAMI spec may require .02" of jump to lands, having less will not cause a pressure spike. I've loaded 0.005" of the lands with no problem and as I understand .005-.01" is common for precision reloaders.

Someone please correct me if I'm wrong.

What it sounds like to me is ptg has a problem making the reamer to grendel spec. just my opinion

As I understand it, just hearsay, PTG doesn't believe in the Grendel Chamber, there's a case of we know better.

[JASmith]

...he was going to do what people ask for and make free-bore for heavier bullets. Not sure if it will accommodate the 123gr to 129 or 140 but we'll see. PTG is currently making the reamer, its in process. I will not have them deviate from the original since I do not know if the material being removed will open it up and once the barrel is heated up, will it become less accurate? That is an unknown to me. I know Steve would not do what he is doing if he felt there was a compromise but I'm satisfied with the way it is.

Your barrel will be in good hands and I think this entire issue got out of hand but now the Grendel II will handle heavier bullets from the get go and make everyone happy...

The SAAMI-standard Grendel chamber handles heavy bullets just fine already. Check Lapua's load data (on line and in Volume 1) and you will see that the heavier bullets are all seated at reasonably normal lengths. In fact, the 156 grain Mega is seated to 2.260 inches.

We have heard several statements to the effect that the reamer in question was cut to the SAAMI line with a plus and minus .001" tolerance. If this interpretation is correct, this makes, one average, half of the reamers out of spec on the tight side. Why? The SAAMI spec is minus .000 and plus .002" so the spread is right, but the datum is too tight by .001" and will get tighter with wear and resharpening.

The confusion is easily to understand those of us who might not be intimately familiar with SAAMI standards.

[Tedward]

The SAAMI spec is minus .000 and plus .002" so the spread is right, but the datum is too tight by .001" and will get tighter with wear and resharpening.

Thanks for the minus .000 and plus .002 info. That makes sense. Good info....

[JASmith]

You're welcome!

Also, a question from BjornF16 got me to thinking about the effect of allowing the throat to be just .001" tighter than spec.

From the SAAMI Drawing:

• Dia at start of throat = .2644"
• Dia at front of throat = .2617"
• Position of start of throat = 1.5478
• Position at front of throat = 1.7025

This makes change of diameter = (.2644 - .2617) = 0.0027"
and the change in length = (1.7025 - 1.5478) = 0.1547"

Now, suppose we are .001" tight, that makes the change in diameter to the front of throat = (0.0027 - .001) = 0.0017"

We get the new distance to front of the throat through geometry:

L = .0017 x (.1547/0.0027) =0.057"

That means that folks who have throats that are on the minimum side of the presumed reamer spec will need to seat their ammunition about 0.060" shorter than they would with a throat with the minimum SAAMI spec diameter.

This difference could explain a lot of what we are seeing in the way of symptoms.

The corrective action I would ask for is to get my throat cut to the SAAMI spec.

Folks are welcome to wildcat and offer for sale any configuration their liability insurance is willing to cover. I would submit, however, that blaming the symptoms of an out of spec throat on an already excellent performing chamber is not necessarily a good marketing strategy.

[Lead Chucker]

You're welcome!

Also, a question from BjornF16 got me to thinking about the effect of allowing the throat to be just .001" tighter than spec.

From the SAAMI Drawing:

• Dia at start of throat = .2644"
• Dia at front of throat = .2617"
• Position of start of throat = 1.5478
• Position at front of throat = 1.7025

This makes change of diameter = (.2644 - .2617) = 0.0027"
and the change in length = (1.7025 - 1.5478) = 0.1547"

Now, suppose we are .001" tight, that makes the change in diameter to the front of throat = (0.0027 - .001) = 0.0017"

We get the new distance to front of the throat through geometry:

L = .0017 x (.1547/0.0027) =0.057"

That means that folks who have throats that are on the minimum side of the presumed reamer spec will need to seat their ammunition about 0.060" shorter than they would with a throat with the minimum SAAMI spec diameter.

This difference could explain a lot of what we are seeing in the way of symptoms.

The corrective action I would ask for is to get my throat cut to the SAAMI spec.

Folks are welcome to wildcat and offer for sale any configuration their liability insurance is willing to cover. I would submit, however, that blaming the symptoms of an out of spec throat on an already excellent performing chamber is not necessarily a good marketing strategy.

This explains one of the stoner barrels I have perfectly. It is .035" short in the throat! With a dummy round, a 123gn amax touches the lands at 2.212"

[Drifter]

This explains one of the stoner barrels I have perfectly. It is .035" short in the throat! With a dummy round, a 123gn amax touches the lands at 2.212"

2.212 + 0.035 would put you at ~2.247, which is the approximate COL of factory Amax. I think another 0.010 to 0.015 to keep factory ammo out of the lands would be ideal. The 123gr Amax doesn't seem to mind a jump.

I have an older cut-rifled Satern where the 123gr Amax is touching the lands at ~2.262, which has worked very well with ~2.250 COL.

[dammitman]

i just relooked at this whole thread and klicked on the link to the PTG reamers,,,,,,,,,they only cost 138.00??? thats cheap.

[Lead Chucker]

2.212 + 0.035 would put you at ~2.247, which is the approximate COL of factory Amax. I think another 0.010 to 0.015 to keep factory ammo out of the lands would be ideal. The 123gr Amax doesn't seem to mind a jump.

I have an older cut-rifled Satern where the 123gr Amax is touching the lands at ~2.262, which has worked very well with ~2.250 COL.

Right. If the throat was cut .045" to .050" would be just right!

[BjornF16]

JASmith...you beat me to it. I was going to run through that exact exercise tonight.

Bill Alexander: The SAAMI spec reamer has plenty of clearance, it was designed to be extremely tolerant of bullet design, to the extent that many accuse it of being a machine gun chamber. The tolerance for the reamer is not specified within SAAMI only the minimum chamber size. The tolerance is the toolmakers descision.

Up to the toolmaker and chamberer to ensure the chamber is within spec...

I would respectfully submit that these Liberty barrels are below SAAMI minimum chamber size and need to be opened up.

[Kikn]

(Just want to say I'm not trying to drag anyone else into this conversation at all)

I only read this and thought it might be beneficial to the discussion in some way or another from a technical aspect.


With that being said I was looking at a post that Adamlilja posted in another section about barrel cleaning and I was reading on the website and found a couple of articles that were written a few years back that seemed to shed some light on a few things.

http://www.riflebarrels.com/articles/barrel_making/rifle_barrel_accurate.htm

This one in particular below is really about Throat angles (Enjoy)

http://www.riflebarrels.com/articles/bullets_ballastics/throat_angles.htm

[rebelsoul]

Y'all keep it going. I know nuthin' bout gunsmithin', but I like this "Cliff Notes" version. I'm learnin' by accident.

[NugginFutz]

(In my best Irish brogue) Ahhh, so it's Goonsmithing ya want ta learn, now, is it? Well, laddy, you've come to the right place, so long as you don't mind learning with the rest of us.

[customcutter]

(In my best Irish brogue) Ahhh, so it's Goonsmithing ya want ta learn, now, is it? Well, laddy, you've come to the right place, so long as you don't mind learning with the rest of us.

NF, thanks I needed a laugh this morning.