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HBA Cause 4 K9s is a registered non-profit in the state of Colorado (ID# 20131451644)
Suomi M31 80% Receiver AND Lower Siderails - Original ID
Quantity in Basket:
Shipping Weight: 1.00 pounds
Receiver comes with Template installed. (Front stubs come uninstalled to allow cutting out of Mag Well).|
Tube comes threaded for the rear cap, front shroud lugs are cnc cut and the ejector is installed
Remaining tube work to be done includes, opening of magwell, ejection port and trimming of the front stub to fit the barrel (key or Keys if using the Tommer Design)
Tube lowers require final shaping and drilling of pin holes along with welding of the lowers and the filler stub to the finished tube
The lugs for the shroud to lock "on to" are machined and then hand fitted tight here at the shop - this is done so that weather your shroud
is brand new/over size in spec and tight or worn/under spec and loose you can finish the lugs to fit your particular shroud. Yes - this means
most people will need to remove a slight bit of material by hand from the rear of the lugs to get their individual shrouds to "lock" in place
however this also means MOST ALL shrouds can be fitted very well to our tubes where as a "One size fits all" approach would be tight for some a
nd loose for others. I understand that some people may see this as a inconvenience but with the provided reasoning I do hope it can be seen as a benefit by most.
The same logic was applied to the barrel socket - it is intentionally about .006 undersized - this means most will need to do some light sanding
in the socket (or barrel shaft) but it also insures a good tight fit when the build is done. To ream the socket to fit most/all barrels would mean it would fit many barrels poorly.
It is a small amount of work to be done but both instances will lead to a much better firearm once it is built.
The M31 Soumi OID tube sets are intentionally .034 nominal oversized from the original ID
(.017 axial tolerance increase - or about the thickness of 3.5 sheets of average
quality printer paper). This increased tolerance does not alter function of the
design other than it allows air to bypass by the bolt as it recoils verses
forcing the air out the rear cap vent. HBA has produced hundreds of M31 Soumi
tube sets since taking over the design in 2008 (Not to mention the countless
tube sets produced by the original manufacture, Prexis, before our acquisition
of the design and manufacture rights. Prexis production stretches back 10+
years) and have yet to have one come back as failing because of this increased
tolerance. In fact some have even commented that it seems to increase
reliability and smoothes of the bolt travel / felt recoil. The design
modification works because the increased tolerance is held in check by the sear
/ hammer pushing up on the bolt from the bottom and the underside of the site
base (the two ribs that pass though the tube) pressing down on the bolt from
above - this holds the bolt in check from a top to bottom relation, the site
base ribs in conjunction with the rear cap probe and spring help hold the bolt
from yawing in a left to right. Additionally weather it be a NFA (Open bolt) or
Semi Auto (Closed bolt) configuration the nose of the bolt never retracts past
the rear trunnion wall face - simply put the bolt is propelled directly rearward
by the discharging round and is then driven straight forward by the recoil
spring, there is nothing to become "Hung up" as the bolt cycles rearward and
then forward again.
As these are built to order, we ask that you allow 4 to 8 business weeks from receipt of payment for shipping to occur.
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The information/products in these web pages are not intended to be used to construct illegal firearms.
It is the sole responsibility of the purchaser to know and understand all applicable Federal, State and local laws before placing an order.
By placing any order you are agreeing that you accept all legal responsibility for the products purchased and their subsequent use.
For legal semi auto and dummy gun information, contact BATF at:
Every reasonable effort has been made to ensure that all photographs, descriptions and specifications on this website are accurate.
However, inadvertent errors may occur, and changes in design or materials, due to our continual effort to improve products, may result in some change in specifications before subsequent publications are issued.
HBA reserves the right to modify or change specifications with or without notice at any given time.