Just went through this about 5 weeks ago. 

Had an 8" pipe plugged with a muskrat nest (at the time, didn't know what was in it). 

First tried a 12 ga. shotgun 4 or 5 times, then a 22-250 ... 3 times.  NO LUCK. 

Ended up making a corkscrew out of 3 foot of 1/2" rebar and welding that on the end of 60' of 1/2" rebar.  (Over lapped the sections of rebar about about 12" when welding them together). 

Corksrew was made by heating the rebar with a torch and wrapping it around a steel post driver. 

Used channel lock pliers to twist the rebar into the nest.

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The nest ended up being about 6-8 feet through, built of mostly reeds canary grass. 

The first good corkscrew set yeilded about 1/2 of the nest and had to be pulled out with a 4wd 4-wheeler -- couldn't even get the corkscrew unwound out of it to pull a smaller chunk by hand -- thought I had really messed up when my redneck puller 'stuck'. 

The good news was I finally got the entire nest out -- the bad news was it took right at 4 hours of hard physical work (in the mud and the muck -- remember the mud from a few weeks ago).  The closer I got to the upper limit of the nest, the harder packed it got.   NOT fun at all.