UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, DC 20549
  
SCHEDULE 13G
Under the Securities Exchange Act of 1934


Mistras Group, Inc.
(Name of Issuer)

Common Stock, $.01 par value
(Title of Class of Securities)


60649T107
(CUSIP Number)


December 31, 2017
(Date of Event Which Requires Filing of this Statement)


Check the appropriate box to designate the rule pursuant to designate the rule pursuant to which this Schedule is filed:

☒
Rule 13d-1(b)
   
☐
Rule 13d-1(c)
   
☐
Rule 13d-1(d)

*The remainder of this cover page shall be filled out for a reporting person's initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No.            60649T107             
        
1.
NAME OF REPORTING PERSONS
I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY)
 
Broad Run Investment Management, LLC (46-1010523)
2.
CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)
☐  (a)
☐  (b)
3.
SEC USE ONLY
 
 
4.
CITIZENSHIP OR PLACE OF ORGANIZATION
 
Delaware/USA
NUMBER OF
SHARES
BENEFICIALLY
OWNED BY
EACH
REPORTING
PERSON WITH
5.
SOLE VOTING POWER
 
1,568,115
6.
SHARED VOTING POWER
 
0
7.
SOLE DISPOSITIVE POWER
 
1,568,115
8.
SHARED DISPOSITIVE POWER
 
0
9.
AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
 
1,568,115
10.
CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)          ☐
 
11.
PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9
 
5.5%
12.
TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)
 
IA
 
CUSIP No.            60649T107              
         
Item 1.
(a)
Name of Issuer:
     
   
Mistras Group, Inc.
     
 
(b)
Address of Issuer's Principal Executive Offices:
     
   
195 Clarksville Road
   
Princeton Junction, NJ 08550
     
Item 2.
(a)
Names of Persons Filing:
     
   
Broad Run Investment Management, LLC (46-1010523)
     
 
(b)
Address of Principal Business Office, or if None, Residence:
     
   
1530 Wilson Blvd, Suite 530
   
Arlington, VA 22209
     
 
(c)
Citizenship:
     
   
Delaware/USA
     
 
(d)
Title of Class of Securities:
     
   
Common Stock, $.01 par value
     
 
(e)
CUSIP Number:
     
   
60649T107

Item 3.    If This Statement is filed pursuant to ss.240.13d-1(b) or 240.13d-2(b), or (c), check whether the person filing is a:

(a)
☐
Broker or dealer registered under Section 15 of the Exchange Act (15 U.S.C. 78c).
     
(b)
☐
Bank as defined in Section 3(a)(6) of the Exchange Act (15U.S.C. 78c).
     
(c)
☐
Insurance company as defined in Section 3(a)(19) of the Exchange Act (15 U.S.C. 78c).
     
(d)
☐
Investment company registered under Section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8).
     
(e)
☒
 An investment adviser in accordance with s.240.13d-1(b)(1)(ii)(E);
     
(f)
☐
An employee benefit plan or endowment fund in accordance with s.240.13d-1(b)(1)(ii)(F);
     
(g)
☐
A parent holding company or control person in accordance with Rule 13d-1(b)(1)(ii)(G);
     
(h)
☐
A savings association as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C.1813);
     
(i)
☐
A church plan that is excluded from the definition of an investment company under Section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
     
(j)
☐
Group, in accordance with s.240.13d-1(b)(1)(ii)(J).
CUSIP No.            60649T107              
      
Item 4. Ownership.
   
  
Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.
   
(a)
Amount beneficially owned:
   
 
1,568,115
   
(b)
 Percent of class:
   
 
5.5%
   
(c)
Number of shares as to which the person has:
   
(i)
Sole power to vote or to direct the vote 
1,568,115
   
(ii)
Shared power to vote or to direct the vote
 0
   
(iii)
Sole power to dispose or to direct the
1,568,115
 
disposition of 
   
(iv)
Shared power to dispose or to direct the 
0
 
disposition of

Item 5.
Ownership of Five Percent or Less of a Class.
   
If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following ☐
   
  
Not applicable.
   
Item 6.
Ownership of More Than Five Percent on Behalf of Another Person.
   
   Not Applicable.
   
Item 7.
Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
   
  
Not applicable.
   
Item 8.
Identification and Classification of Members of the Group.
   
  
Not applicable.
   
Item 9.
Notice of Dissolution of Group.
   
  
Not applicable.
   
 
CUSIP No.            60649T107              
   
Item 10.
Certification.
   
By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having such purpose or effect.


SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.


Date: February 14, 2018


By: /s/ Bryan H. Adkins                 
Bryan H. Adkins,
Chief Compliance Officer
Broad Run Investment Management, LLC


The original statement shall be signed by each person on whose behalf the statement is filed or his authorized representative.  If the statement is signed on behalf of a person by his authorized representative other than an executive officer or general partner of the filing person, evidence of the representative's authority to sign on behalf of such person shall be filed with this statement, provided, however, that a power of attorney, for this purpose which is already on file with the Commission may be incorporated by reference.  The name and any title of each person who signs the statement shall be typed or printed beneath his signature.

Attention.  Intentional misstatements or omissions of fact constitute federal criminal violations (see 18 U.S.C. 1001).