Chamber Throat Questions

[archistrong]

Hey All,

I completed my second AR15 upper build (first for the Grendel cartridge) and I think that I have an issue with the chamber throat length. My build definitely falls under the "budget" category but consists of the following:

• Spinta Stainless 20" 264LBC Barrel (rifle gas length)
• Spinta Type I Bolt (.125" depth)
• M16 Bolt Carrier
• Generic Upper Receiver (used but in good condition from local gun shop)
• Generic Quad Rail (15")
• Micro Gas block and stainless gas tube

During the build I tested the chamber dimensions w/ a set of Go / No Go gauges and it checks out. Thinking all was good I finished the assembly and set about checking how much bullet "jump" there was with the typical COAL of 2.26". I took a piece of virgin brass (PPU PRVI) and cut two slits in the neck. I opened the neck enough so that a 123gn Sierra HPBT would slide in / out easily but still maintain its position. I set the bullet very long and then chambered / ejected the round. Holy cow was that thing set back deep! It measured 2.112"!

I repeated this several times thinking that I did something wrong but it came out to within +/- .005" each time. I then tried chambering a factory round (123gn PPU) to see if it was the loose neck that was giving me a false reading. When I pulled the round it wasn't set back as far (2.21") but had very pronounced marks from the lands. I've reached out to the guys at Spinta for comment but haven't heard back.

As I see it now I have a few options:

• Say "heck with it" and shoot it anyway. I don't like this idea as having the bullet jammed hard into the lands scares me due to a unknown spike in chamber pressure. Not sure what this does to accuracy.
• Seat bullet to -.001 of measured throat depth (2.111"). This means the bullet is set nearly .15" deeper than the load data recommends. I worried about what this will do to chamber pressure and accuracy.
• Open the throat with a PTG Uni-Throater. I have some experience with machining and feel relatively comfortable with this considering, from what I've read, it's typically a methodical process using a hand reamer w/ bushings from PTG. Still, this type of gunsmithing is uncharted territory for me.

If anyone on here has any experience with a short throat on their Grendel barrel or with the folks at Spinta I would appreciate some insight into this. I read a bit about some people discussing short throat issues with their Grendels but I got lost at the Grendel vs. Grendel II differences. I hope I posted this in the right forum, figured it could fall in a few different forums as well. Anyway, thanks again!

[LRRPF52]

I would use a Manson true SAAMI 6.5 Grendel reamer to take the chamber to the correct specs if you felt the barrel was worth salvaging. Is it concentric and square? What type of steel is being used at those price points?

I just saw this barrel company's name come up recently, so I am unfamiliar with them. By the time you have invested in the tooling and labor, you're into the price of a barrel from a known vendor with a solid reputation.

[archistrong]

Looking at their website, the barrel appears to be 416R Stainless. I put the barrel on a 4-jaw chuck and compared the chamber / barrel O.D. for concentricity and checked to make sure that the chamber face was square, all is good there. I probably should have checked the threads for concentricity as well but it was getting late and I forget to.

I picked it up when they were running a 30% off deal on their barrels (usually run $240). At this price point I have a little wiggle room to mess with it before I feel like its money down the drain, especially if I can rent a reamer from a place like 4-D Reamers ($33 to rent a 264 LBC-AR reamer). I knew there might be issues with a barrel at this price point and am willing to do some work to bring it into spec. I will say though, if this process starts to look a bit too daunting I'd rather return the barrel / bolt and go with something from the established smiths as you suggested. So far this still seems like something that's still in my wheel house. Thanks!

[LRRPF52]

One thing that I know for sure is that the SAAMI chamber flat out shoots. I've compared barrels where the only difference was the chamber, and I can rapid fire 5rd groups into the .3's with a SAAMI chamber.

Some have 8rd groups in the .2's.

[archistrong]

An update now that I have had a chance to talk with the Sam Spinta. He thinks that the chamber / throat was cut too short. He is having me send it back to him and they'll take a look at it and ream it into spec. For their part, they have been VERY quick to respond and are working to make it right. I'll report back once I get the barrel back from them.

LRRPF52 - It's accounts like yours that have drawn me to this cartridge and hope to be putting rounds downrange by Thanksgiving!