Need an education- Chamber and throat

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Mike6158, Dec 14, 2009.

  1. Mike6158

    Mike6158 Well-Known Member

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    First let me assure any real gunsmiths out there that I'm not going to start hacking on a barrel or rifle. I just want a good understanding for why some things are the way they are.

    The thing that got me to wondering about this topic is my VTR (.308) and the extra long bullets that I have to load to seat a 150 gr Sierra Pro Hunter .010 off of the lands. The rifle really likes .010 but the bullets are .184 longer than COL (Sierra manual). I understand the COL isn't really a hard and fast length and I don't mind shooting or single shot loading the rounds however my little pea brain thinks the VTR chamber must be pretty sloppy. So then I got to wondering if somehow the chamber could be opened up into the throat area (decreasing distance to the lands?) and then the back of the barrel trimmed off. My pea brain says hell no... because it's not a flat surface and a few other reasons... but a gunsmith I'm not. Can this be fixed without replacing the barrel or do I just live with it?

    How do "premium" barrel makers handle this? It seems to me that the above problem, if it really is a problem, could be handled when the barrel is machined. I get the whole "gotta make them sloppy for factory ammo" thing but this is ridiculous. A factory bullet is big time short compared to my .010 bullets. A lot shorter than the Sierra manual recommendation too.

    Also- I don't pay much attention to COL anymore. All of my bullets are seated using a MIC gauge and typically I am seating to some dimension from the lands.
     
  2. little black 243

    little black 243 Well-Known Member

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    I am no gunsmith by any means but I had a similar problem with a rem 700 25-06. I had the barrel set back 1/2" and rechambered with a shorter throat. Cost me 130 bucks. Seats the shorter bullets fine now.
     

  3. Mikecr

    Mikecr Well-Known Member

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    It's likely Remington could care less about reloading specs implied in some manual..
    I've had several factory guns that lands could not be reached on in cal (no bullets existed long enough). Pretty sure it's common.
    But why would they care?
    Do Sierra specs reflect Remington's standards? Or Winchestor's?
    It's likely that neither are SAAMI, but instead whatever the makers (and their lawyers) want for THEIR ammo.. And that's probably unreachable lands.

    Also, premium barrel makers do just that.
    They don't do alot of barrel finishing(chambering, headspacing, crowning, engraving, coating, etc). That's best left to your gun builder, with desired reamers and action in-hand.
    Your VTR is Remington's design and build.
    If you want it YOUR design and build, then who do you figure is gonna make that happen?

    A gunsmith might be able to reset the chamber, if it happens that he has a matching reamer. But if he has to order one, he'll probably recommend another cartridge or new barrel.
    Ya just can't add metal.

    Many people demand more than factory providers are bound to abide.
    Some will later concede that going custom from the git-go would have saved them time & money.
    Some go Savage cuz barrels are plug and play with them.
    If you really really like your gun, and no load is gittin it, just rebarrel it and be done for about $700.
     
  4. BountyHunter

    BountyHunter Writers Guild

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    Understand your chamber throat is set by the reamer design, not the barrel manufacturers.

    Most "factory" chambers are set at max SAAMI and with the mftrs "most common" bullet for that caliber.

    It is normally impossible to have the same throat set for 150s and 200s both at .010 off the lands.

    If your throat is short, PTG sells a hand throater that can be used to lengthen the throat. Go slow removing .005 at a time and recheck 2-3x.

    Most guys building custom guns will load the bullet they want to try and shoot at either max magazine OAL if a mag gun or at the best location in the case for that bullet and send the dummy case to the reamer mftr to let him spec the reamer of that dummy. Reamers only cost less than $150 and you get to keep it. Pretty smart move on a $3000 plus gun build IMO.

    BH
     
  5. Aldon

    Aldon Well-Known Member

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    BountyHunter,....or for that matter any of you knowledgeable folks:)

    I appreciate the information on the ideal way to proceed to build a rifle and get a custom reamer built.

    How would one go about doing that for the 284 Shehane.

    Is it a little reversed? Order a reamer, have the dies custom made, etc...

    Does the situation change when you do not have the ability to load a round in the particular wildcat?
     
  6. Mike6158

    Mike6158 Well-Known Member

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    Sorry for taking so long to get back to this... been a long week(s)...

    Bear with me a little... I'm just trying to learn about chambers / bullets / etc

    If I am having to seat extra long to get to the lands wouldn't that mean that the throat is long?

    Bear in mind that the rifle I'm talking about is shooting 3/8 moa groups at 300 yards. This isn't a thread about what am I going to do to "fix it" or even if I want to "fix it". It's more of a theoretical look at what I could do if I wanted to. And a chance to gripe about factory rifle chambers :D

    Back on topic-

    Do bullets vary that much? It looks like they do. I've been reading Bryan Litz's book and looking at the bullet tables. Specifically the ogive radius values for the same weight but different brand bullets and the difference surprised me. I can see why it might be hard to ream a chamber to fit everything well.

    Lets say that I decided to stick with 2 manufacturers (Berger and Sierra for instance) and keep the weight between 150 gr and 168 gr. Would it make sense to go to the trouble to have a reamer made and have a gunsmith ream the chamber and set the barrel back to give me a little shorter bullet? I'm thinking the answer is no, not really. Not for this gun anyway.

    I would probably have better luck by having a barrel fitted to the action and tossing the triangle in the corner. On the other hand it shoots pretty good so why fix what isn't really broken?

    I've already put more into it for the stock and optics than the gun cost so until I wear it out I'm probably better off being happy with tight groups and long bullets unless someone has a better idea.

    Sorry for typing out loud... and rambling...
     
  7. MagnumManiac

    MagnumManiac Well-Known Member

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    Although the throat of a chamber is cut by the reamer, they differ and are not bullet specific, in your case your throat is long. You do not need a reamer made, what you need is a throating reamer which is different in that once the chamber is cut it is used to obtain the correct throat length as to your specs.
    The only way to shorten it is to remove the barrel, cut the desired amount of threads from it and re-thread/re-chamber with the throat length that you want.
    This is easily accomplished by measuring your mag box and setting up a dummy round to suit and instruct your GS to cut the chamber so that there is .010" clearance to the lands.
    This is neither difficult or expensive to do, but it will make the stock to barrel fit look ugly in most instances.

    If you're happy with the way it shoots and the calibre it's currently in, then just shoot it until you need a new barrel and make the new one to your specs when needed.

    Unfortunately you will never get the perfect throat length for all bullets types and styles.

    I must admit that I have never found a long throat to be detrimental to accuracy because there is always more than one sweet spot with every bullet I've tried, one up close to the lands and one further away from them.
    gun)
     
  8. Mike6158

    Mike6158 Well-Known Member

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    I ladder tested (per a link someone posted in another thread) with bullets seated .010, .050, .090, and .135 off of the lands The farther away the bullet was the worse the rifle shot. I suppose I could work my way back to Sierra's recommended COL and see if something shorter shoots as well as the long bullet.

    The VTR has a triangle shaped barrel so removing threads would probably make for an ugly (or uglier depending on your viewpoint) duckling.

    The entire thread may become moot soon. I have some Berger VLD's for it so the process is about to begin again. Who knows... maybe it will come down to needing to chose another bullet.
     
  9. Mike6158

    Mike6158 Well-Known Member

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    Progress...

    I watched an old AGI video on how to thread a barrel and ream a chamber... I understand now. Thanks for all of the replies.
     
  10. woods

    woods Well-Known Member

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    I didn't see this touch on so thought I would bring it up.

    IMO, the longer throats are also a safety margin for the gun manufacturers in that when you have a longer throat you have a larger combustion chamber. With a factory load the longer throat will develop less pressure (and less velocity) than one with a shorter throat.

    For instance I have a 338RUM with a custom rebarrel that I sent a dummy load to the gunsmith and said to throat it so that this bullet will just touch the lands. Now I can seat a little shorter than that, be close to the lands and still fit the magazine. This means that it has a much shorter throat than a factory gun. This gun will expand primer pockets at max loads in manuals. For example the max load with 225 gr bullets is 98 gr RL25 but I can only shoot 94 gr RL25 and have the case hold a primer. Still get the book velocity at 94 gr so that is not any big deal.

    Tight chambers and short throats develop more pressure (all other things being equal) than loose chambers and long throats. So the gun manufacturers use the longer throats as a safety margin.