I recently posted this in another thread in response to someone asking how to choose the "best" PDW stock.  Here are some factors important to consider.  I am doing my best at keeping it unbiased and not product specific.  Hope this will help those trying to decide and understand the differences of the various PDW Stock on the market.

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Some things to consider when choosing a PDW Stock. "Best" should be qualified by best in what? Best in most compact? Best in lightest? Best in most comfortable? Best in most compatible? Etc. 

Many PDW Stocks looks the same with the classic two guide rods but they are all very different if you inspect it closely. 

1. Overall Stock Length - Not all PDW Style stocks collapse to the same compact length. The shortest, fully collapsed, is about 4.75" and some are 5.5" or greater. If overall length and concealment is a main concern, make sure you get the shorter stock. 

2. BCG Compatibility - Many PDW Style stocks requires you to use their proprietary BCG. Usually BCG and buffer is together as one piece. Some allows you to use a standard M16/AR15 BCG, with or without an adaptor. 

3. Ability to "Shotgun" the Upper - As in #2, those that use proprietary BCG/buffer combo means you cannot shotgun your upper receiver without removing both pins. Some that uses an adaptor with standard BCG requires special step to hold the bolt back in order to shotgun the upper. An extra step in the standard / normal operations to learn and remember and train for. 

4. Guide Rod Construction - Not all guide rods are made of the same material or the same diameter. Beware of soft guide rods that may bend or roll. Deep cuts in the the locking notches can also weaken the rod. Some use 4140 steel. High impact resistant steel like S7 tool steel would perform better. 

5. Stock / Buttplate Material - Some are polymer and some are made of 6061 or 7075 aluminum. Cost will vary based on the material used. Typically, polymer is least expensive, then 6061, and 7075 being the most expensive or highest strength. 

6. Metal to Metal Contact & Sloppy Fit - One of the biggest problems with the guide rod design is metal to metal contact. There must be a gap/tolerance to allow the rods to slide in/out reliably so it usually feels sloppy. When dirt and debris get in between the steel rod and the hard anodized aluminum stock housing it will feel gritty as it grinds the dirt in between the metals. The more you use it the looser it gets. Guide bushings helps separate the metals and keeps the dirt out. Almost all PDW Stocks do not have bushings. 

7. Cheek Weld - One of the most important job of a stock is to provide good cheek weld. Some Stocks do not have a cheek rest, but a gap between the buffer and the buttplate. Usually the end of the buffer tube get in the face/cheek of the shooter. If you want a good cheek weld, make sure your stock have a cheek rest. 

8. Overall Weight - Not all Stocks weight the same. Typically PDW Style stocks are heavy compare to traditional telescoping stocks. Some are 30+ oz down to under 20 oz. 

9. Aesthetics - it is usually not on the top of my priority list but it is a factor to consider. We each have our own style and preference. 

10. BUFFER Design - This ties into #2 and one of the most critical parts of a PDW Stock. The buffer should allow you to shoot 5.56/.223 reliably. Pending on the gas system, the gas port some times may need to be opened up. The overall weight of the buffer is also important as well as the reciprocating mass that dampens bolt bounce. Be careful that some buffers are not compatible in 9mm blow back type setups. A different buffer may be needed from the manufacture. 

11. Latch Placement - the placement of the latch is important. Pending on how it is designed, it may be easy or difficult to deploy. 

12. Locking Mechanism - the locking mechanism should be robust. Preferably locking both guide rods. Some stocks only locks in one direction, not both directions. 

13. Stock Positions - Some stocks offers only fully open or fully close. Some offers 4 positions or more. Choose what works best for you. 

14. QD Attachments - Some offers no QD sling attachments, some offers one, some offers two QD points. 

15. Lower Receiver Compatibility: 

  A. If you have a lower with built-in QD points at the rear, you may be out of luck, most if not all of the PDW Stock guide rods will not fit. 
  B. If you have an ambi bolt release on the right side of the lower, many PDW stocks will not fit neither as the guide rods are too long and will crash into the release. Some stocks have uneven length guide rods to clear the right side bolt release. 

16.  Safety Selector Interference - A common problem with many PDW Stocks.  The guide rods (while collapsed) will ride just over the safety selector lever with little to no gap in between to articulate the lever.  Some PDW stocks have a larger gap between the safety lever and the guide rod due to better rod placement and/or smaller diameter rods.  Althought it may not be 100% out of the way, there is enough gap to still work the safety.  Some do not allow that space.  When the stock is fully extended, this is usually not the problem.

Any chance you'll be following g up with a comparison table for all of the stocks available using the parameters you laid out here?

We like to stay neutral on comparing the stocks between manufacturers.  A comparison chart is best come from a unbiased 3rd party.  We would be happy to provide the specs and facts on our products.